This is a CEDET error message, so has nothing to do with completion-UI.
> I tried M-TAB after M-x completion-setup-semantic , but i was told that "No
> tags table loaded; Try M-x visit-tag-table"$B!#(B
It sounds like you haven't enabled Semantic in the buffer. I'm not a
Semantic expert, but you'll probably want to enable
`semantic-idle-scheduler-mode' at least.
If it still doesn't work, try getting Semantic to work in the buffer
first, before trying to enable completion-UI. I recommend reading the
INSTALL file that accompanies CEDET carefully...