A 50mm enlarger lens focal length is 50mm (duh) or about 2 inches for non-metric people like me. That means your lens should be about 2 inches from the negative plane to achieve proper focus.
I am going to assume you have the Omega Super Chromega Dichoric d2 light head mounted on a D-II/D-2 enlarger frame/base. To get your lens to focus you need a flat lens board for 50mm lenses. If you use the 2-3/4 inch lens cone (or the much longer 4-1/2 inch lens cone), you can't get the lens close enough to focus. Here's a good resource for what parts do what:
http://www.khbphotografix.com/omega/Enlargers/D2.htm
As a quick reference, the 2-3/4 inch cone is for 135mm lenses and the 4-1/2 inch cone is for 150mm lenses. If you're printing 135 film (35mm), you'll probably never need lenses with these focal lengths unless you want to print
really small.
If you have a different enlarger base than a D-II/D-2, then the lens mount might be different. Others may be able to help there as I have limited knowledge of the other D-series Omegas.