Q: Errr Why is there no gui??
A: GUI to a server software?
Q: How do I add UF_DELETE to one of my users
A: "lundftpd -e" or "SITE setflags"
Q: I can upload but not download, or, I can download .txt files but not others?
A: Usually this is due to fiddling with the quota values in lundftpd.conf, or setting a users quota to 0. Leave the lundftpd.conf setting alone, and set users to -1 if you do not care about quotas.
Q: How do I set it so the users don't have access to the whole disk?
A: You can make the root directory to be somewhere lower (see lundftpd.conf), instead of being the whole thing. The easiest. You can also setup groups, then assign a user to that group, and that he can only see directories belonging to that group.