The "app store" is a paying service. The community modules themselves are free, but you have to install them in other ways (manually). Read this question and its answer.

Particularly this passage:

The Apps Platform allows users to install modules easily in 1 click, making sure that the correct version of the correct module as well as its dependencies are installed properly. The Apps Platform is a paying service because it requires maintenance and bring convenience. That particular service is available for free for databases with less than 2 users in order to make it easy for people who evaluate Odoo to install additional modules. If you have more than 2 users and you do not wish to pay for this service, you have the option to download, import, and install modules yourself manually by going to Odoo's development platform and taking them from there. It is currently being migrated from launchpad.net/openobject to github.com/odoo/odoo. The list of all the modules is available here: www.odoo.com/apps.