I use ubuntu 11.04.
In my taskbar no extra symbols are shown like for skype or vuze but instances still run in background ( ps -e in terminal shows them).
How can I fix this problem?
This is not a problem - it is a configuration setting.
Updated as per your comments:
Ahhh - I guess you have the newest format by default. If you want those icons you need to start Unity in 'Classic' mode.
The alternative is to follow the instructions at Ubuntugenius, the simplest version being to type this in a terminal:
and then refresh Unity