well then that's it .... just put the .pbo in your addons folder (or the Resistance addons folder, the .pbo readme will tell you which). Sometimes you have to restart OFP to make it appear. The addon will now be available inthe appropriate place (Units in the mission editor for example) and you will be able to play missions that use the addon.
The location of you addons folder will depend on where you installed OFP .... but its probably
c:\programfiles\codemasters\operation flashpoint\addons
just cut and paste the .pbo from the desktop or unzip folder or whereever it is into that directory and away you go.