Add us to your circles on Google+.
this would be due to a combination of your smf template (i.e. a minimum or fixed width) and the width of your mambo main section. You'll have to adjust one or both to sit correctly (I recommend a variable width smf template and a wide main section)
The default SMF theme is actually variable width (to a limit on the amount it can shrink) however it sounds like your mambo template is very narrow
Page created in 0.15 seconds with 14 queries.Page served
by: 10.0.100.134 (10.0.100.111)