QuickTime X can't use .srt files
I strongly recommend Subler, with which you can not only soft bake the subtitles but easily add metadata as well. This allows playback with subtitles on iTunes and iOS devices as well, and even swiftly converts .mkv files for playback on these devices as well.
Small tutorial:
- Drag a movie file on
Subler, then drag the .srt-file on the newly opened window - Optionally choose a language and set as default to autoload
- Optionally click the search icon on the top right to add metadata
- Click the share icon right next to the search icon to merge the files