.DMG Mounting Woes

I am trying to get the program FrameThief, for simple animations.
I download it & its a .SIT file, requiring Stuffit expander. Okay, I go to download stuffit, which is a .DMG file, but it wont even finish downloading before it cancels. Not recognized.
I try going into HD-System-Library-Core Services-Image Disk mounter, to see if there is a setting that needs to be tweaked, but when i double click the icon it wont open!
What am I missing?
S.

I tried downloading firefox, as per the stuffit troubleshooting guide. It is also a dmg file & wont fully download. I keep getting the message "mounting failed"
S.