I have just installed 7.1.0.336 to my 9360, and the mobile hotspot option isn't there!

Do I have to activate it somewhere? I have searched a lot and didn't find anything.

In the Device Version it says: 7.1 package 1133 (v7.1.0.336)

Hoping you know the answer.

PS. Sorry for my english, it's not my native language.

Telcel may not have provisioned your account to allow tethering. If they have not, the option will not be available.

The OS I installed is from O2 UK. Still do they have to allow tethering?

It doesn't matter from what carrier the OS came. What matters is whether the carrier on which you are using the device allows it.

Check with them.