What are you using for download?
Try the original link from the mbrepostery. It should work fine without any weird popups.
Check out where your browser saves your downloads, if it isn´t there do a virusscan.
If it´s negative, turn on adblocking/popupblockin in your browser and download it from the mbrepostery (stickified download thread). Should work like a charm.
This mod is for free, and anyone selling it to anyone does it illegally and shall be punished. If you notice such an attempt, please drop a note at Saxondragon´s. I don´t think your forced winzip purchase has anything to do with MnB but rather with your pc - you´re either having a weird virus or spyware or even, worse, your OS extracting .zip files automatically using winzip and an out of date trial version. So either update your winzip to something free or click the skip registration now, do it later instead of closing it. Now that I think of it, that´s more likely the case than a weird virus infection.
Does this happen each time you download a .zip file or is it exclusively for PoP?
Go download some free packing software 7zip and make it your standard extraction application. Then redownload PoP.