I swear by Feliway (which someone mentioned earlier)... They make a spray and a plug in, it's a pheromone that helps calm them down when they are threatened by environmental changes, etc.. It is a little pricey, but it absolutely works!!

Also, the rule of thumb for litter boxes is to have 1 more litter box than you have cats (even though they were fine before kitten came along).. It's best to space them out throughout the house as well so they have options on where they want to go.. I know it sounds like a pain, but generally fixes inappropriate urination problems....

I hope this helps, and I hope the kitten doesn't have to find a new place to stay........