Yes, you have to set it manually.

It wasn't there when you negotiated your contract.

 

Quota = However much Mb you are allocated per month. ('500'Mb in your case)
Per ['1'] Month(s)
Start Month Day (The date your plan started, when you first used your phone)
Warn me over ('75%' is recommended IMO)
Refresh ('every hour', as i want to keep battery usage low)
Notification (Up to you, if you like to regularly keep track, then 'always visible')