migration assistant won't find my backup, please help!

I have been keeping time machine backups for both my MacBook Pro and my iMac on the same partition of an external hard drive.

Migration assistant won't find any of the backup series that were done by plugging the drive directly into the computer (eg through backupdb files located in the top level of the drive).

It only finds the backup series that was done over the network (eg stored as a backupdb file inside of a sparsebundle located in the top level of the drive).

Unfortunately my iMac backup is the one I need to recover from, and this backup was only done through a direct connection to the hard drive, and so is only available as a backupdb file located in the top level of the drive..

My MacBook Pro was backed up both directly (by manually plugging the external drive into the power book) and also indirectly (by sharing the drive over the network while the drive was plugged into my iMac). The sparsebundle created by the backups through the network can be found in migration assistant, but the backups created through the direct connection cannot be found.

All three of the backup series (iMac backupdb, MacBook backupdb, and MacBook sparsebundle) can be accessed through time machine, but only the sparsebundle backup can be accessed through migration assistant.

Can someone please help me access the backupdb backups with migration assistant?