I'm not an A/C expert so don't want to give you bad advice. The refrigerant cans should have instructions on them, or you could call the service manager at the KC smart center and get the procedure from him (he goes by Tiger Wong on SCoA.)
I found this on SCoA; let me know if it is accurate:
On a 2008 smart, the high and low pressure ports are located together on the passenger side of the service compartment right next to the passenger headlight. The high pressure port is about one inch above the low pressure port and the best way (I found) to plug your gauges into the ports is by taking the passenger front wheel off and utilizing the access hole in the fender skirt.