Hi,

First, you need to register the SID. This is a unique system identifier which makes it easy for siemens to provide support (one of the perks). 

Register SID

Second, you need to put in the SID in the multiproject settings (right click Multiproject->Properties. There is a column to enter the SID.

Please note:

Up to PCS7 V9.x, the SID is not mandatory and the only thing you get is an annoying popup, but as of PCS7 V10, the SID is mandatory in order to run a project.