the first thing I would try is to delete the Dreamweaver Configuration folder. See the Common installation issues pdf for details on locating the configuration folder:
common_installation_issues.pdf
In the same location you find the DreamweaverCS5 folder, there will be a Extension Manager CS5 folder as well, delete the configuration folder under that path also, this should hopefully correct the issue with the Install button in the extension manager being greyed out.
if you continue to have problems with the extensions manager, you should contact adobe support to get it sorted out.