you cant,used to be easy in 80's,piece of garden strimmer wire would suffice,now though the 'coin mech' is a sophisticated bit of kit,i.e,detecting bad/fake currencies etc.
You cannot add Facebook Credits for free. They must be purchased with a credit card or via PayPal to Facebook. They do occasionally give away free Facebook Credits though.
There are many sites in which you can play online slot machines free of charge. Most of these sites do not offer prizes for playing. Some of these sites may offer certain contests with prizes.