While it is hidden, Revit files do have an origin. If you haven't messed with the templates too much the origin should be at about the intersection of the elevation symbols in the plan views.
There are various methods of linking in files in Revit. The basic premise behind using origin to origin when linking is so that the links don't move. If you use some other positioning method like center to center, if there is a significan change to the architecture file the "center" of the file may change, which could cause the linked file to move, which is disasterous to an MEP project.
To start the linking process from the Insert tab, click Link Revit.
When the file open dialog appears browse to where ever your link (Architectural) file will be stored and select it. DON"T CLICK OPEN YET!!!
Notice that near the bottom of the dialog there is a control that says "Positioning:" This is the control that you need to set to "Auto - Origin to Origin" before you insert the link.
After you have specified "Origin to Origin" click OK. This will automatically place the link so that the origin in the host (MEP) file is aligned with the origin in the link (Architectureal) file.