The Vehicle Identification Number (VIN) is required by an MOT station when the time is due for your fortwo's annual checkup. However, if you take the car to an MOT station that isn't familiar with smarts, they may struggle to locate the VIN and ask you. For your information, the VIN in the fortwo is located in at least two places. Certain markets also have a third position at the bottom right of the windscreen (when facing the front of the car):

The two places where all fortwos have VINs are as follows. The first place to check is the left side door jamb (LHD: driver's side; RHD: passenger side). The VIN in these photos has been obscurred for security.

The VIN on all fortwo 451s starts with 'WME451...'. However, if the VIN plate is missing from the door jamb, there is a second, more permanent place you can check.

Open the boot, lift the carpet and pull the black retaining bung so that you can expose a bit more of the chassis. The VIN is located in the area circled.