Hi Alexej,
after being tipped off by Tijs that there might be a problem, I have tested the installation too but for me it all works fine. The steps I performed were:
- downloaded eclipse classic 3.6.1 for Linux 32-bit (I'm on Ubuntu)
- setup proxy settings (because I'm behind one, this might differ for you)
- added update site for Activiti Designer. Unchecked the "Contact all update sites" checkbox to prevent Eclipse from contacting the default Eclipse download sites which tend to be very slow
- checked the designer feature and installed
- restarted Eclipse when prompted
This worked without problems, so for the moment I can't reproduce your problem.
Tested this on 3 different systems, 2 of them Ubuntu 10.04 and one Ubuntu 9.04