In order to get Urchin working for Plesk. It should run as the same user as the apache.
http://help.urchin.com/index.cgi?&id=1243
To do this, the template (urchinwebd.conf.template) should be the one that’s modified, NOT THE REAL ONE (urchinwebd.conf).

Lines that need to be modified should look like this:
Before:
User nobody
Group psaserv

After:
User psaadm
Group psaserv

Valid for Plesk 7.5.