I know that I may be just too stupid, since I just installed Bob's mod for the first time.

I installed all of Bob's mods except for the adjustable inserters mod. I started a new game and now I cannot build any long fast inserters. They also do not appear anywhere in the tech-tree as a result of researching a tech.

What am I doing wrong?

All inserters can be made into "long" now by doing the research for long inserters.

Needs Bob's mod: https://mods.factorio.com/mods/Bobingabout/bobinserters

There are options in this mod that will allow you to place any inserter as long now.