This looks like it could be a pain. Searched the Evilution site http://www.evilution.co.uk/index.php and all it has it how to fill the tank depending on your model of smart.
If you can find / see / reach the crack, you may be able to put some silicone or epoxy on it. There is no pressure in the tank so a water sealer should work good on the plastic. Looking at a picture tho, the likelyhood you will see the leak is low. I will have to pop off the hood and poke around to see how difficult it will be.
Good luck and let's be careful out there,
I guess you could add the Smart to the List...
Washer tanks are known to eventually crack and leak in pretty much all cars as the years go by. Some are more prone than others but it should ordinarily be a relatively simple job. I just replaced one, yesterday, in a Toyota Corolla... literally. Some might crack in a year or two, others might crack in 10.
The tank is a hassle to get out apparently - not like on some other cars. One person on SCoA had success patching the crack in the tank with EternaBond Tape - 10 bucks at Walmart: http://www.walmart.com/ip/Eternabond-Microsealant-Tape/10825191 I'd try that before spending 500+ bucks for a replacement.
The washer tanks necessitate removing the radiator and lower paneling under the car to access it. It would be a good idea if you need to spend that kind of money to get other jobs in the same region taken care of... I would have had those radiator hoses replaced under the same labor bill...