I have downloaded the trial version of the CS6 Master collection twice now. The download gets to 100% and before it says it has completed it starts the download process again.
The file that downloaded is “MasterCollection_CS6_LS16.dmg.adadownload” and does not open.
That's just the encoded file, not a normal DMG. Anyway, try these suggestions:
For anything more we need much more info like system specs, browser used, location where you save to and so on...