I bought this Dimension Pro 1.5 vst in May and have been trying since then to install it, I'm using Ableton 9. It won't accept the serial number. I've tried, literally everything, and looked literally everywhere on the internet to find a solution. Today I've given Cakewalk 30 days to resolve it or I want my money back. I'm afraid it should just work, I shouldn't have to be finding solutions.

This is a user forum, you should probably take this up directly with tech support as techead suggested here. The Cakewalk contact information is here. It would appear that DimPro is not supported on OSX 10.8 according to the system requirements page.

I'd like to see them support recent OSX version, but I just don't see that happening. On OSX at least, Dimension and Rapture look like abandonware. Sad because I love those synths, but w/ Logic X requiring Mountain Lion, their days on my machine are numbered.

Unfortunately, I don't think you stand much chance of getting a refund, though. From Cakewalk's web site:
 

Dimension Pro System Requirements

The following are the minimum and recommended system requirements for Mac and PC
 
 
Mac: (minimum)

• OS: OS X 10.5 or higher (OS X Lion & Mountain Lion are not supported)
• Processor: Intel Core or newer processor
• Memory: 2 GB RAM
• Disk space: 8 GBScreen Resolution: 1024 X 768, 16-bit colorAn Audio Units or VSTi capable host application