Unfortunately, there is no version of PhotoMontage Pro that works on the iMac with OSX 10.10.5 installed because the product appears to be discontinued. You'll have to use other tools that were designed as alternatives to PhotoMontage Pro.
The best tool that I can recommend for this particular job is Adobe Premiere or After Effects. These are both video editors and video producers and although they might require a bit of knowledge, I am sure that the outcome will be the expected one.
