The lack of FTL does not mean trade is impossible, you just have to think in longer terms. Since there is no FTL, yet the aliens are travelers, it is safe to assume they are a patient species who think in the long term. Possibly beyond their own life spans. It is even possible they work as a linear hive mind where multiple individuals make up their consciousness. If one dies, it can be replaced by a new one giving a sort of immortality. Look into A Fire Upon The Deep for this sort of entity. Another possibility is from Diaspora where they are effectively immortal, can alter their sense of time, and change their sense of what they find fulfilling ("I'm going to reprogram myself so the endless tedium of interstellar travel is FUN!").
Why did the alien come to Earth?
There are two possibilities, it was on purpose or it was not.
If it wasn't on purpose, it could have been...
- a disaster such as a crash
- a layover for fuel and supplies
- they happened to be passing by and noticed signs of intelligent life
Crashing on a primitive planet, they may choose to enter sleep until humanity reaches a level of technology capable of making the supplies necessary to fix and fuel their ship. The device would open when it felt their manufacturing and base scientific abilities were up to task. For example, a person crashing in 500 AD and needing a new laptop battery and screen, even if they knew how to make one, would have to wait for enormous advances in material science alone to accomplish it.
If it was on purpose, it could have been...
- to develop trading partners (like I said, they're patient and think very long term)
- to develop military allies ("these Humans sure to reproduce fast, and they're good at fighting", this is the premise of the Yehat uplifting the Shofixti or the Asgard attitude in Stargate)
- to have somebody to talk to, a different point of view
- to spread their religion to
- to use as slaves or vassals
- a sense of obligation to educate the savages
- bring Humans around to their way of thinking before they become a threat
- to avoid common disasters brought on by technology
- to avoid polluting their neighboring systems (this is the premise of Quarantine)
- it was all a prank
- it was all a mistake
What knowledge is in the device? What is the key discovery we need to find in order to open it?
The key is tied to what is in the device. The key would indicate Humanity is ready for the contents of the device, either to understand the content, or use it responsibly, or the conditions are so that they need it to save them.
Since they're so intertwined, I'm going to mix together keys and discoveries a bit.
- a benchmark indicating we have reached a certain level of technology (when the Monolith buried on the Moon is exposed to sunlight it knows Humanity has achieved space flight)
- we have the destructive power to breech the device
- a certain dangerous technology is discovered (atmospheric levels of industrial pollutants indicate we're on an arc to destroy our environment, fluctuations in the Sun's energy output indicates we've begun harvesting plasma which will eventually lead to a nova) and the device contains how to make the technology safe, or advice to avoid it, or a safer alternative, or a warning beacon for the aliens to come save the day
- a puzzle which requires a level of linguistic and mathematical advances to solve indicating we're now worth talking to
- an indicator that society has achieved a certain benchmark of responsibility (I'm not sure how the device would detect this)
- instructions on how to join intergalactic society (a beacon pointing at the nearest outpost, instructions to build a communication device, rules for trade)
- there are enough Humans and we're ready for harvesting
- the knowledge to create a valuable material or item which the aliens will come to take or trade for
- we've achieved a level of technology where we can manufacture spare parts and fuel, it contains instructions to do so, we're now a colonial fueling station
- their religious texts
- military technology and propaganda about how evil their enemy is (which could backfire due to cultural and moral differences, "they allow their enemies to surrender and don't kill their wounded, they have no honor!")
- we've developed to the point where we've become a threat, it's a bomb
- the instructions require special devices to read: it's emitting radio waves; it's just under the surface and visible in x-rays; it requires knowledge of higher dimensional physics
- it's a special device and the aliens have hidden it on a backwater planet where nobody would think to look (a la The Fifth Element)