Assuming you have installed everything in the normal areas, the filepath should be:

C:\Program Files\Microsoft Games\Flight Simulator 9\Captain_Sim\navigation

That folder should contain the following files:
.DBs:
Airports
Airways
Fixes
IAPs
Navaids
Runways
SIDs
STARs

.LDBs
NavData

.CFGs
b757

The folder should also contain a 'routes' folder for saved FMC routes.

If it doesn't, the best fix would be to remove all 757 components and reinstall.

Cheers,
Kyle