I've not used ZA Pro, only the free version, & not had much networking experience, but I did have some fun & games with getting the free 1 to work when I had a mate's machine over here to sort out.
When you say ZA won't allow your other machine to accept files, do you mean that you're trying to access the firewalled machine from the other 1, or trying to send them to the other 1 from the firewalled 1? If you're trying to access the firewalled 1 from the other 1 then there's a couple of things you can try:
1. try accessing the firewalled 1 from the other 1, then on the firewalled 1 go into ZA'a log viewer & look for an entry corresponding to your other PC. Hopefully you'll have an entry with an IP that's something like 192.168.xxx.xxx [It's possible I could be remembering the numbers wrong - I tend to forget stuff if I've only done it once or twice

]. Anyway, my point is, you can right click on that entry in ZA's log where it says it's blocked it, & tell it to Add to Zone -> Trusted. That might solve your probs.
2. Goto the Firewall section in ZA, then the Zones tab. Click the Add button, then IP Range. When it asks you for the IPs to enter, for the start of the range take the 1st 3 sections of your other PC's IP address, & put a 0 on the end, & for the end of the range, do the same as the start, but make the 4th section 255.
Hope this helps.