The house builder program is external to the game, I'm assuming you have an empty plot you want to build on ingame...
What you do is double click the house sign while standing on the plot, there are different pages of options, you want to find the page that lets you customize your house then click on that option and you will go into customize mode. This is special mode of play, you are invisible and unattackable while in this mode and cannot leave the house plot. To get out of customize mode you can right click on the gump that gives you wall/floor/etc options.