Assuming that you installed the asset, if you can remember how long ago you downloaded the template guide, then set your Content Manager filter in the Search Box to show Installed=True and sort the filtered set by "Installation Time".
Look for the name of the asset (which it seems you have forgotten?) at the approximate time-frame you remember downloading it.
Then, assuming the asset reveals itself to you in this search, open the route you are working on and look for that name in the appropriate Toolbox (Add Asset/ Add Trackside Object, etc.)
(Hope this helps - that's the method I've used to locate a previously downloaded asset whose name I had forgotten by the time I needed to apply it.)
Sometimes I wish Trainz Asset authors would use better, more descriptive naming conventions than some do... There are some hidden gems out there, just waiting to be discovered.
Cheers! PC