I have Squirrelmail 1.4.5 running on FreeBSD 4.11-STABLE. I installed compatibility-2.0.2 and successfully applied the patch for SM 1.4.5 as described under INSTALL. Then I installed custom_from-1.1-1.4.0 and successfully applied the patch to the SM source as described under INSTALL.
After logging in, I go to Options->Display Preferences and can see a radio button labelled "Use Custom From When Composing Messages" Yes/No at the bottom of the page. I set this to "Yes" and click submit. But when I go to compose a message, no From field is displayed, just the usual compose page.