Hi! You have structural problem that's why the sidebar is slipping to the bottom. First of all you need to have a fixed width on the #primary div (now it's 100%) and also a float:left; attribute.
The #secondary should also have a fixed width and a float attribute in the CSS file.
Changing this will brake the template (the footer will be misplaced) but you can fix this by adding a clear:both; (either into a br or p) right after the #main div (or directly adding in the CSS file - line 512 - a clear:both attribute).
Finally if you want to have that white rounded background to act as a background for the whole main wrap then you should configure it (via CSS of course) on a container div (for example #main).
Hope this helps.