You generally need to purchase the right size negative holder, a suitable lens and the right lens mount for that lens.
EDIT - I just realized I was wrong when I said you could enlarge up to 4" x 5" with that enlarger. The maximum size is actually 6 x 9 cm with a B5 - still large enough to include 6 x 7.
Here are links to KHB's pages for the Omega B5/B6: http://www.khbphotografix.com/omega/Discontinued/B5.htm
and the somewhat related Omega B7: http://www.khbphotografix.com/omega/Discontinued/B7.htm
as a comprehensive resource, KHB is invaluable, as is Harry Taylor's site Classic Enlargers: http://www.classic-enlargers.com/