Yup, to clarify, only the SUP files included with Project Threepio support positioning, and they are the recommended format for that and other reasons. If you just got the SRT files, such as are included with some preservations, I’d recommend downloading the whole project. The SRT’s will do in a pinch, but it’s really not the best way to go.

I’ll PM a link if you haven’t found it yet.

EDIT: SUP files are supported by all software players I’ve tried, if you mux them into the video (instructions for this are included in the README). Without muxing–i.e. just specifying an external subtitle file, like you do with an SRT–I’d recommend MPC-HC.