Cannot install Pro Sites, Anti Splog etc.

When I try to install Pro Sites I get to a page which asks me to download and then it returns me to install then download then install then download etc etc in a continuous, stupid, infuriating, silly loop!!!!! The same happens with Anti Splog and I don't have endless time to waste checking out the others but I expect they are all the same. The plugins do not appear in the list of those available in the stupid form I am filling in now so I have chosen 'User Activity' instead, which is the first one on the list.