Hi TrachyPithecus and welcome!
First of all, it doesn't sound like you have Microsoft Office SharePoint Server 2007 (MOSS) and InfoPath Forms Services (IPFS) which goes with MOSS. If you aren't using SharePoint to store your XML, your MAC users won't be able to submit the forms (because there is no InfoPath for MAC, the only solution is IPFS which requires MOSS, aka SharePoint). Your PC users will need InfoPath to submit the form if you don't have MOSS/IPFS. The only feature supported in InfoPath for pcs without InfoPath is showing them a read-only version (filling out the form, sending e-mail and submitting the form from a pc without InfoPath will not work unless you have MOSS/IPFS).
If you have MOSS/IPFS, you will need to approve the XSN (InfoPath template), and make sure it is set to be "browser-enabled", before it can be used from MAC and PC desktops. We can show you how to do this, but you'll have to purchase a support pack. Describing how to do it in the forum (or reading MS literature) is a bit complicated and not guaranteed to get you going.
Anyway, let us know if this helps.
Cheers!