My plug ins files is situated in wordpress-content/plug ins/site/apptivo/apptivo.php , Now i would like activate the apptivo.php wordpress plugin.

This can be a really unclear question. May be the wordpress plugin not turning up inside your listing of installed plug ins? Try moving the the apptivo/ folder from site/ into just plugins/, ie. wp-content/plugins/apptivo/... (I am unsure if getting an additional directory between plug ins and also the specific wordpress plugin will screw it up or otherwise).

Should you just don't understand how to install the wordpress plugin, then yes, browse the user guide. Its quite simple. See your dashboard, (wordpress/wp-admin/) and login should you haven't already. Consider the sidebar around the left. You will find a lot of options, included in this is 'Plugins'. Click it, and you will be come to a listing of the installed plug ins. Then simply just find 'Apptivo' and activate it.