I'v just installed the scidb chess database, and I can't figure how to play against an engine from there.
Is this feature even supported ? If it is, how to use it ?
On this page, you can see that Scidb is not intended to be used to play against an engine.
You can use
scidb for game analysis (click on the train button in the board tab),
chessx to play against the computer (and game analysis too) and
raptor chess interface or
Jin to play on FICS (Free Internet Chess Server). These softwares all work on Linux. Other softwares are availble on Windows and OS X.