Move over to Debian., Mandrake = slow and anoying (windows like) for me, but u may like it.
On debian I did
apt-get install ethercap
and it grabbed the latest files off the net, installed them and configured them for me an 43 seconds.
If you still wanna try www.google.com/linux
is you friend. place parts of that error in there, you should find something, also ethercap-gtk is seperate package, may need to get that.
Sorry I wasnt very helpful