Downloaded rom for usb loader but can not get WBFS to merge it

I have my Wii all set up with usb loader and such, and im on the last step which is to get the rom onto my usb drive.

I already checked my usb drive and it is compatible with the wii and usb loader. I have formatted it to Fat32 and loaded it into WBFS. I have also downloaded a Gamecube rom from the download list above (Metroid Prime). After downloading this, I attempt to use WBFS to merge it, but the file shows Ion memory as 1.67g, but the estimated memory as 0.00g?

When I attempt to merge the game anyway it loads instantly and seemingly does nothing.

Ive used both 7-zip and winrar to extract the file out of download, but both have the same result.

Not sure where i'm going wrong here, hopefully someone might know what to do.

Thanks.