Try with avidemux 2.6
.18, found here: http://www.avidemux.org/nightly/
If you are using Video Output [Copy] setting, subtitle filter can't work.
In subtitle filter, load the srt filter.
You should be asked
This is a srt file. Convert to SSA ?
It should now be showing in the right column (Active Filters)
Click [Preview] Right hand bottom.
Move slider forward to 3 seconds with the provided sample srt file attached to this posting.
In the main GUI mark [v
] Play filtered. subtitles should be showing when video is played.
when this all works, you can save video with MP4 Muxer, subtitles should be in the saved video
YouTube Avidemux Demo: Subtitle Hardcoding in to video