You can do it with an FTP Proram, like FileZille...
Select public_html
Select the Oxwall folder instalation
Select ow_plugins
drag and drop the plugin folder.
then in the plugins admin panel
select
Available Plugins
Select te Plugin and press Install
THe other way is in the Plugin admin
Select ADD NEW
Open a browser window and select the Plugin
Up load it.
The same... go to Availabel Plugin
select the plugin and press install.