How do I add freon to a 2001 Nissan Frontier Crew Cab SE?

Got No heat....went and bought Freon from local auto parts store and i screwed the blue cap on the front of the engine but the connectors won't go in?  Do i need a special connector? or is it another connection i need to be looking at?   Also, my fan only turns on high....low and medium do not work...any ideas?

Thank you,

Sean