When you demux with pgcdemux you have to select 'create Celltimes.txt' but make sure you unselect 'Include end time'.
Pgcdemux gives you the chapters in Celltime.txt.
Now when you mux with muxman you need to select 'file' on the top left side of muxman and then 'Import chapter'. Then you browse for the Celltime.txt that pgcdemux created. Then import all the streams you want to mux. Thats it.