Q: icloud photo stream on my upgraded MacBook Pro problem

i have recently upgraded my laptops to mountain lion and have tried to open the icloud photo stream setting in system preferences. 

it will not let me carry out the tick, unlike other options

trouble is, i have two macbooks and i can't do it on either of them

is there something fundamentally simple that i haven't done

i would reaaly appreciate the solution to this

many thanks