my personal hotspot it's not enable

I've got Iphone 4 and I was O2 pay monthly costumer but now I'm with GiffGaff with unlimited Internet and 1Gb data for connection with shearing PC and tablet.

My personal hotspot it's not Enable and when I wanna set up personal hotspot I receive one massage from O2 and writing in the box (To enable Personal Hotspot on this account, visit http://o2.co.uk/iphonehelp/modem) Go to Website Cancel

So could you please help me for set up my personal hotspot or enable this one.

many thanks