There should be a diffuser between the lamp and the condenser assembly, is that in place?
Also, the knob behind the lens that swings the red filter into place turns easily, and may have the filter impinging slightly on your image.
Another thing to try would be the position of the bulb, the socket holding the bulb can be adjusted up and down, to center the lamp.
I found a manual for your enlarger on this site, which may help if you don't have one;
http://www.jollinger.com/photo/cam-c...Durst_M600.pdf