Installing XP on latest MacBook OS X
If I understand correctly, you just need to install VMware Fusion first, which will allow you to install and run Windows XP as a virtual machine, so that you can install and work with that important software of yours.

If you still have a backup copy of your old XP virtual machine (it's a large single file, if I remember correctly), you should be able to run that. Again,  after you have installed VMware Fusion.

Installing Windows using BootCamp is a very different thing (dual booting) and is indeed not supported for XP anymore.