You need to open the cue file in a text editor, such as notepad, and:
Press control+A to select the whole file
Press control+C to copy
Then click in the top box in Zag and:
Press control+v to paste.
The Guide (here or in the install folder:
viewtopic.php?p=1952#p1952 )
is your friend.
I did not understand your comment about the DTS encoder. Are you making a CD or a DVD and what is your source?
If you are going from a CD source, to a DVD you'll need to resample your source to 48KHz, and set plogue to process at 48 Khz, so you'll end up with 48 KHz mono files for the encoder. Audiomuxer is one tool that can do the resampling.
Z