Hi Petzi
As far as i know XMBCbuntu is denying to execute programms for every user by default. I have no idea why. So far i know only how to change that for each programm individually. But in this case it should do since you only need flirc to run i guess.
Anyway to do it, enter the XBMCbuntu GUI. Right click on Flirc. Go to properties or settings (sorry i'm at work and don't know the exact words but you should get the idea). And now you should see an option where you can change the rights for executing this program to everyone or only your user. After that you should be able to execute it.
As i said i'm at work, but if you need closer information just let me know an i'll post the exact way.
Cherrs!