To load a (La)TeX file directly in jPreTeX,  first open a document (by "Ctrl o" or by clicking on the open file button or by choosing "open a document" in the File Menu) ;
then click on the combo "Files of type" and choose "TeX files" ;
now choose the file you want.
    jPreTeX will translate this TeX file (*.tex) in a new jPreTeX file (*Bis.joc) and open it. You can modify this document then preview it as usual. This will create a new TeX file *Bis.tex.