Installation problem - can't see the plugin at all!!!

I have a fresh 3.1.1 installation with no plugins active (some have been installed, network activated and then deactivated). But although mult-domains 1.1.3 is in the plugins directory, I cannot see it on the Plugins page either under Network Admin or any of the Site Admin's.

The server is completely under my control, the permissions are 2771 for directories, 0640 for files and all the other plugins are visible (and appear to work if I activate them).

I have downloaded it a few times and also tried adding to plugin/wp-content/mu-plugins (this is for the supporter plugin so I don't think it is right) but it still can't be seen.

Am I missing something?