Hey there, welcome to the forum and glad you're enjoying the game. As to your questions:
1) Ammo packs can either be unloaded from the ground (stand on top of them and press 'U'), but in that case you only gain normal ammo from them. To use the reload function, put them in your prepared slot (press 'e' to access the equipment screen, go to the prepared slot and use tab or enter keys to select the pack).
2) You can indeed only ever have one Master Trait. The Cleaver grants the
effects of the Blademaster trait, it doesn't grant you the trait itself - even though, stupidly enough, it does say so on the wiki.
But not for much longer...Allow me to take this opportunity to direct your attention to the
Quick Questions thread, which is very useful if you're unsure about any aspect of the game. Your eagerness is appreciated, but please try to only log bugs if you're sure the game isn't doing what it should.