Hi Arachn1d, welcome! The first thing you should do is download the Bitcoin client on each computer. It will need to download the block chain, which might take an hour or so. In the meantime, explore the forums and find threads about mining. There are a few mining programs available, the only one I've used is the Python OpenCL Miner and that was with an nvidia card. You'll basically run the mining program, and point it at your running bitcoind client. You can do this with the CPU, GPU and GPU simultaneously, although the macbook will probably never generate any bitcoins. Ask in IRC if you need additional help: http://webchat.freenode.net/?channels=#bitcoin-otc

I will stop mining with my mac book pro instantly and rather save the environment electricity. 

I wouldn't use the macbook at all or the CPU on the gaming rig.
Yeah it will help a small amount but the power it will use won't pay for it's self and if you increase your hashing by .5% it's not worth increasing your watts by 20%.

Those are just guesses and not that close but the input output would be something horrible like that on the CPU.