Finally decided to buy Designer 3. But I can't use it. (yet???)

According to Carvewright the minimum requirement for Mac is OS 10.3.

I have 10.5.8. Neither Designer 2 0R 3 will work. System says the program will not operate on this architecture.
Anyone else have Designer 3 working on 10.5.8 or earlier.
This is not a Maverick or Yosemite issue. I have another Mac with 10.6 on it. The program works on it but I don't want to use that computer. I also do not want to upgrade the OS from 10.5.8 to 10.6 for a hundred other program issues.

Has this been addressed previously? Thanx for any and all insight.