What kind of file was it, Hans? It's quite possible that you'll have to install it the old fashioned Lightroom way, not using extension manager or whatever it's called. My guess would be that it's the former.
That site has been around for a few months but most of the real plugin authors have stayed away. We've got our own established ways of reaching users, the site's uploading process was very time consuming, and we just didn't like the deal (no trial versions and a heavy annual charge before you sell anything). So the Lightroom part of that site mostly consists of worthless preset packages at extortionate prices.
And they are not "Adobe plugins". They are all third party, and any Adobe review is at best cursory!