Goto Control Panel (Windows Control Panel) -> Add/Remove Programs -> Plesk for Windows -> Change -> Plesk setup will pop up asking whether to Modify, Repair or Remove -> Select Modify and choose Tomcat in list (not installed by default).
Tips:
Remember to check your SMTP setting after Plesk is done with installation, I had it rewrite config (instead of my domain name, it has put "home" and mixed up Reverse DNS blacklist), as well as putting it's default skeleton over mine...