Viewing *.HPG or *.PLT

[Platypus]

Hello "Xn",

I have hundreds of either *.HPG or *.PLT file that I would like to convert the PDF format. But first I would like to (re-)view these drawings!
After downloading and installing the program I thought it would be as easy as just browse to the exact folder location and have a look at the thumbnails to do my review. However, I can not see the HPGL/2 drawings on my screen! Am I doing something wrong? Hope to hear from you, Regards, Adri