Open ComicRack.exe and you will be presented with the main screen which is defaulted to the Library tab. (At this point ComicRack may attempt to find all eComics on the machine, if so just let it finish).
Click on the Folders tab and navigate to the desired folder in which your comics to be converted are located. You will then be presented with the covers of all available comics in that folder in the right hand window.
Right click on any of the comics in the right hand window and choose the 'Select All' option to highlight all available comics in the folder.
Right click on any of the highlighted comics and choose the 'Export eComics -> Convert to CBZ' option. A window may appear asking you to confirm the action, if so just click Continue.
ComicRack will now begin batch converting all of the comics in the folder to CBZ. The process should not take long (approx. 1 second per comic) depending on your machine and the number of comics being converted.