How can I re-install Mac App Store
There isn't a stand-alone Mac App Store updater. Your best bet is to reinstall Mac OS X 10.6.6, using the combo update. It's a 1.06 GB download, but it contains all the parts of prior updates (the 10.6.1, 10.6.2, 10.6.3, 10.6.4 and 10.6.5 updates) and should repair your current install.