Click on "Tools" then "Hardware Configuration". Monitored House Code is mid right under the blue boxes.
NOTE: AHP version 3.204 has bug that will reset this to the house code you have the most modules / macros regardless of what you set the HC to be. Unchecking "Check Monitored House Code Usage" under "preferences / ActiveHome Pro" has no effect on this bug.
e.g. I use HC "M" for most of my macros but want monitor HC "C". Since I have more Macros in "M" than I have modules in "C", AHP resets to "M" every time I download. The only way I found around this was to create a room called "DUMMY FOR BUG" and create 15 "C16 modules" to make HC "C" have more things and keep my monitored HC at "C"
I've discussed this bug elsewhere on the forum but I think the above sums things up.