As said right there, thats your problem then, 3.1.3 on the 3GS cannot be jailbroken. (well actually it can, but only a few people have managed to get it working)
Reason being is 3.1.3 changed the bootrom, the only way to do it is to downgrade the bootrom, and alot of people when they do that lose connectivity so i'd leave it tbh.