Hi subtitles are fun, and I read you needed some more help.
You are very close! So that’s nice, but we’ll see if I can explain it well enough 
Let’s get this started with the two most important things:
- Following the Plex Naming Scheme especially the part about Adding Local Subtitle Files.
- Possibly doing The Plex Dance to clean things up.
I’ve found that SRT subtitles are the best.
And the Avatar example used in the link I gave you is what works.
Once you do get them recognized and loading, you might find a better playback experience if you upgrade to Plex-1.2.0, because our friends have been working on subtitle support a lot. If things are still not as you prefer (color, placement, size), consider using Plex Media Player which has better subtitle support still.
Good luck!