This is a weird problem. I have installed WP Mu via SVN but had too many folder permission problems and the widgets under appearance -> widgets were all expanded and did not have the add to sidebar option.
I then did a new install and ftp the files and installed again, same problem. This was with WP Mu 2.7.1 so I cleaned it all out and installed with WP Mu 2.7.0 same problem. Then upgraded 2.7.0 to 2.7.1 same problem.
Here is a screen shot of the widget page in the admin area:
I have 2 other WPMU sites on the same PowerVPS dedicated server and they are fine however I migrated them from a reseller account and so they are not fresh installs on this server. So it is possible that there is a permission problem under one of the WP Mu folders or files?