First of all, your MPN ID must be authorized to sell Microsoft Online Services. If you have signed the Microsoft Online Services Partner Agreement and taken the assessment, you should be good there.
You must also have enrolled your MPN account in the Microsoft Cloud Essentials program located at www.microsoftcloudpartner.com.
This site is where you will be able to take advantage of your 250 Free Office 365 Internal Use Rights (IUR).
It also turns on the partner features for your Office 365 account.
Once the features have been turned on, follow the screen shots below to start sending out invitations!