I've downloaded the cs6 trial alright but when I'm running to install the dmg file it starts to extract it but in the end comes an error message saying the following disk images couldn't be opened "DesignWebPremium_C invalid checksum" any ideas why?
The error occurs if the file got corrupted during the download. A slow or intermittent internet connection may cause this.
If the download is complete, you can swith off error checking from the disk utility and try. See instructions to do that in the following thread: http://forums.adobe.com/message/4370247#4370247
Switching off Error checking will ignore minor errors and it just might work. Else, I'm afraid, you'll need to download again.