ssh into the device...
apt-get update (this will pull down the latest available "things" and their location)
you can add things to your apt repository list if the default things available aren't enough.
Things can go wrong though.... installing aptitude is never a bad idea. (other than space)
https://www.mobileread.com/forums/sho...&postcount=103
apt-cache search whatever (where, "whatever" is what you are searching for)
apt-get install whatever
(there are a few posts on this thread of me doing this with examples)
EDIT: HERE
https://www.mobileread.com/forums/sho...4&postcount=29
hope that helps.
Other things:
Rsyncing other profiles on and off the device...
https://www.mobileread.com/forums/sho...2&postcount=48
what I do at the start.. (where you are I think)
https://www.mobileread.com/forums/sho...5&postcount=35