On another (apparently unmentionable) Smart forum, Aaron Lephart said
To the left of the SAM unit is a powered strip (circled in yellow) that has a couple fuses already utilized depending on your smart car options. There are 6 open slots to tap into power from. All you need is the "pigtail" or "holder" that houses the fuse rating of your choice and clicks into the SAM unit.
The Mercedes part number for the pigtail adaptor is 211-545-12-01 and can be purchased at any Mercedes or smart dealership. It's about $6.00. The part number is from a 2003+ E class Mercedes but it is a shared part across the brands.
The fuse block is only restricted by the main fuse size. However I would not use any higher then 30amp.