Question regarding paralells trial

Hello everyone,

I have not got any experience with Apple computers at all,
but me and me friend, who has a Macbook, wanted to play some Windows games together.
Consequently, we came upon this software which sounds great, and which is exactly fit for our purposes.
My friend downloaded the trial and tried to install it, however, he ran into a problem that was
not something we expected at all; the installer would not let him pass the license agreement.

When he had scrolled through the license agreement, and was asked to accept it,
he clicked accept, yet the 'continue' button was still greyed out. The program could not be coerced to let my friend continue the process at all.

I have included a screenshot for reference. We live in Holland, 'ga door' means continue.
As you can see, my friend did indeed scroll down the license agreement, so that can't be it.

I can not provide any detailed statistics on the mac, only that it is a Macbook, and that it is running Leopard,
version 10.5.3 if I am not mistaken.

Has anyone had this experience before? Is my friend doing something wrong?

Thanks in advance,

DiSt