Nobody can tell you anything without proper system info or other technical details. We know nothing about your system, so we can't give you even a basic rundown.
I can't tell you anything about the difference between the tutorial your working on and "your" AE2015 but I can tell you how to set your cach up on an external drive.
go to Edit>Prferences>Media & Disk Cache
in that dialogue you will see options for "choose folder"
just click that, navigate to the drive you want it on, select the folder you want it in (create a new folder if you didn't already) and click ok. That's it.
Choose Edit > Preferences > Media & Disk Cache (Windows) or After Effects > Preferences > Media Cache & Disk Cache (Mac OS), and do the following step :
- Click one of the Choose folder buttons to change the location of the media cache database or the media cache itself.
For more information on Caches, go through the link below :