In V7, you need to go in Settings -> Modules -> Installed Modules, then remove the installed filter in the search box to get a list of locally available modules. There is no way of getting that list from Settings -> Modules -> Apps which always look for things listed on apps.openerp.com.

From then, you can install modules, but from the kanban and form views (the install button is not available in the list view, contrary to v6.1).

If you have shell access on the server running openerp, you can also use the -i module_name command line option to install a module (use -u module_name to update the installation of an already installed module).