I am trying to add 134a to my truck, I bought one of the do-it yourself kits , but the connector seems to small to fit theconnector near the passenger front side of the radiator. Is that the low pressure fitting? Does it require some sort of adaptor?

 

You need freon already? You need to check the actual pressures before trying to add.
That is the high side fitting which is why it is smaller so the can doesn't explode in your hand.
I think the low side fitting is closer to the firewall. Your pressure should be around 35psi when running at idle.

Why do you need to add refrigerant? You sure you don't have a blend door issue? Just checking.