Hi again!
Are really nice guide about how to manage drivers in MDT is Johan Arwidmarks guide, this one is for MDT 2013 but the procedure is the same.
http://www.deploymentresearch.com/Research/tabid/62/EntryId/112/MDT-2013-Lite-Touch-Driver-Management.aspx
Read the whole guide but focus on the Drivers for Windows PE (the boot image) part.
You're windows pe version should be 4.0 which means drivers for windows 8 are needed to be added to it. Don't confuse it with the driver needed for the OS (Windows 7) you are deploying.
Although the Windows 7 drivers for the OS will probably work in WinPE 4.0 as well, but not before being imported
Hopefully you're MDT was setup using this guide or similar. Otherwise its a good idea to start following it.