thanks jeroenn.
sorry, i'm not very technically literate.
which sequence do you propose: clicking on the address as it appears in bitstamp, then hitting simultaneously ctrl and C, then clicking on the window in multibit that's directly below the 'send, request, transaction' line, then clicking ctrl plus V?
doing that copies the address in the multibit window, but still the same '... is an invalid address' pops up...

could it be that i have to switch to a newer version of multibit?
or that my pc OS is too old (i'm using vista)?

is it obligatory to additionally get the bitstamp QR in the appropriate multibit window to be able to send the BTC? so far i find it impossible to drag the QR over to multibit.

and do you know how the transfer fees are handled by multibit? do they automatically deduct some satoshis when sending my BTC?

many thanks!