Did you try this?
1. Go to http://www.oxwall.org/store
, navigate to the plugin's page and download the archive containing plugin files.
2. Go to your Admin Panel > Plugins > Add New section.
3. Click the Choose File/Browse button, select downloaded plugin archive and click Upload.
If that did not work, try this:
2. Unpack the archive and upload extracted files to ow_plugins folder on your server.
3. Go to your Admin Panel > Plugins > Available Plugins section. The plugin you uploaded should appear in the list.
4. Click 'Install' to install the plugin you need.