uhh... you shouldn't be trying to get a current reading across a voltage source. You want to measure the voltage. You want the multimeter on the 'VAC' or 'ACV' setting, that is AC Voltage. I have a cheap RadioShack meter in front of me and it has 200V and 500V AC settings. 200V setting is okay because we shouldn't see any more than about 120V AC. Adjust your multimeter to the correct setting BEFORE you start poking the probes around. Go to a known good outlet and put the probes in the two slots, the slot that is larger is the positive one. It doesn't matter if you get it backwards, you'll just get a negative reading. I don't know what measuring the voltage is gonna tell you if the breaker blows every time you turn the receptacle on. If everytime your flip the GFCI switch it blows the breaker, it sounds like the GFCI receptacle is bad. The only reason the breaker switches off is because of an over-current condition. Without anything plugged into the receptacle, that means only one thing; the receptacle is shorting out internally. I don't think the wires are shorting out anywhere, otherwise the breaker would immediately switch off everytime your turn it back on. Your problem is linked to when you switch the GFCI receptacle on, I say replace it