Inkpress is a multi-feed RSS and Atom reader. If something is not working, the fastest route to a fix is an issue on GitHub with the feed URL that is giving you trouble.
Open an issue on the Inkpress issue tracker. Including your iOS version, your device, and the exact feed URL involved makes a problem far quicker to reproduce and fix.
Open the feed list, choose to manage feeds, and paste the address of the feed you want. Inkpress accepts both RSS and Atom feeds. Most sites publish theirs at an address ending in /feed, /rss, or /feed.xml — if a site advertises its feed, that address is what Inkpress needs.
Inkpress renders what the feed publishes. Many sites deliberately publish only a short summary rather than the full article, in which case opening the entry's original link is the only way to read the whole piece. If an article that does contain a full body renders blank, that is a bug worth reporting.
Entries are cached on your device as they are downloaded, so anything already pulled down stays readable without a connection. New entries still need a connection to fetch.
Inkpress subscribes to one feed on first launch so that the app is not empty. Remove it from the manage-feeds screen the same way you would remove any other subscription.
There is no account and nothing to sign in to. Your subscriptions live on your device only — see the privacy policy for the full detail.
Open an issue on GitHub and describe what you expected to happen and what happened instead.