I’m looking for a CRM system that is compatible with both Windows and Mac.
Any cloud-hosted CRM system like Salesforce will be compatible with both Windows and Mac by extension since you're accessing a remotely hosted service over the internet (ie "the Cloud").
Michael, try BPMonline CRM. As a web-based system BPMonline CRM works in browsers for Mac as well as for Windows.
Test-drive BPMonline CRM at http://www.bpmonline.com/autologin?locale=en or request live demo: http://www.bpmonline.com/company/feedback/presentation