this has been frustrating me to no end so last night I figured it out.

you press the Blue arrow key (the one to assess the blue keys) pres it twice to lock it. now when you click "Sys/Ctrl key" in the top left corner in rDesktop and move the cursor over the position you wish to right click (remove the styles from the screen) and press the "Sys/Ctrl key" once. ( if you press it twice the cursor is locked in right click mode.) - everywhere you tap the screen will be a right click. to go back to left click just press "Sys/Ctrl key" once again.

keep in mind that sometime there is a lag so you can get get ahead of your self.