Your Smart Fortwo only has two fuel-ups and the first fuel-up was marked as partial. When you mark a fuel-up as partial, Fuelly doesn't calculate MPG for that tank and the next one. So you just haven't given Fuelly a full tank baseline to work with yet. You should start getting some numbers on your next fuel-up. There's more info in the FAQ:
yeah, exactly. If you can't fill to the top, you mark it as a partial tank. But say you drive 10 miles and fill it up, that's a full tank. The thing Fuelly needs to know is whether or not you filled it all the way up.