Once you have the -stable version going, you'll notice that in the 'options' file, the version it says you have installed is 4.3-RC or something like that.
If you attempt to install a package at that point, the only way (as far as I can tell) to make it work is to replace that information with 4.2-RELEASE
Use the 'pkg_add -r' option.
Say you want to grab ncftp1-1.9.5.tar.gz from the freebsd servers. To do this, you simply need to type
pkg_add -r ncftp1
It will then grab the correct version based upon your installed tree.