I have a trial of MindManager that I am using on a tablet PC and do not see an option/tab or menu item ot use the pen. Worked in previous trials on this computer but those were not web based, it was Mindjet 12 and the trial was installed on my computer
To enable the pen functionality there should be a pen and mouse icon in the bottom left hand corner of the Mindjet Window. If this is not there, Windows has not recognised the device as a Tablet PC. If you go to Control Panel - Programs & Features - Turn Windows features on and off you find an option to turn the Tablet PC features on. (I can't find this in Windows 8) but you may be on an earlier OS.