That was all explained there:
And the wiki also gives a full list of valid IWAD
files and where they can be obtained; which includes the Freedoom ones. It's all on the wiki.
My guess? The OP doesn't understand the difference between IWAD and PWAD, and didn't bother to read the second paragraph of "how to start ZDoom", which explains how to load PWADs. You'll notice how he talked about renaming his custom wad as the IWAD... He's yet another of these guys who think the IWAD selection box is a full-fledged mod launcher. And he prefers to bitch and whine rather than expand the effort of actually reading what he claims to have read, because that way he can try to pretend to save face, rather than acknowledging there was something he didn't understand.