VSTs

Thanks for that

Yes, 1.5 uses VST effects. If they don't appear, there are several things to consider. Firstly, you have to specify the directory they are in (it doesn't matter where this is as long as you've specified a path to it, and it's on your local machine). Then you have to refresh the effects list (that command is in the effects list options).

If they don't appear at this stage, and these are modern VSTs, you have also to be aware that if they are 64-bit ones, that they won't run in 32-bit Audition 1.5 without some sort of bridge - and I'm not sure what if anything would actually work with something that old now.

Finally, they have to be well-written VSTs. El cheapo ones that took coding shortcuts often flatly refuse to work in Audition, because it's, erm, fussy. There is no avoiding that - back in the day, a lot had to be re-coded by their developers to get them to work properly, or at all.

VST Effects or VST Instruments?

The extension which Audition recognises is .dll but only if the VST is an effect and not an instrument (VSTi).  It is a very long time since I used AA 1.5 but I assume it used VSTs?