assuming that you've added the neurodebian ppa to your system witch is the easyest way to get the psychtoolbox to ubuntu
try to update your packet list with sudo apt-get update
and then try to install again with sudo apt-get install matlab-psychtoolbox-3
if that doesn’t help check your software sources and see if the neurodebian nonfree ppa is installed (all matlab stuff is part of the nonfree branch).
if you got the free branch installed remove it put add the non free branch:
wget -O- http://neuro.debian.net/lists/trusty.de-md.full | sudo tee /etc/apt/sources.list.d/neurodebian.sources.list
sudo apt-key adv --recv-keys --keyserver hkp://pgp.mit.edu:80 2649A5A9
that should do the trick