I assume you're referring to the Relationships Graph? You don't need to add scroll bars; all you need to do to add another table occurrence (TO) is to click the "Add" icon in the lower left-hand corner:
The Graph's scroll bars will automatically appear if the new TO needs more room.
Alternatively, if you want to force them to appear, you can:
1) Drag an existing TO to the right or down until it scrolls off the screen.
2) Zoom in on the Graph until there's not enough room for the existing TOs. Scroll bars will automatically appear when that happens.
also if you are on a newer Mac scroll bars are turned off by default. You can turn them on in System Preferences>General
PS its problematic to think of the relationship graph as an ERD
Well, coh, we could just use the acronym like this: