Hi,
Do you have a link to the datasheet for the coin accepter you're using? Can you confirm you've got the line that will be pulsed connected to digital pin 2 on the Uno? Other than that,
attachInterrupt(0, coinInserted, CHANGE );
...the above will call your function to increment your counter twice as often as (I presume) you'll want to, ie on both edges of each received pulse.
Cheers ! Geoff