Did you use this PPA for Trisquel 7 and follow the directions?
https://launchpad.net/~ubuntu-mate-dev/+archive/ubuntu/trusty-mate
================
PPA description
These MATE 1.8.1 packages for *only* Ubuntu Trusty, they have been side
ported from Debian.
In order to correctly install and configure MATE 1.8.1 on Ubuntu Trusty you
will also need to add the following PPA:
ppa:ubuntu-mate-dev/ppa
Open a terminal complete the following steps to enable the appropriate PPAs
and install MATE 1.8.1 on Ubuntu Trusty.
sudo apt-add-repository ppa:ubuntu-mate-dev/ppa
sudo apt-add-repository ppa:ubuntu-mate-dev/trusty-mate
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install --no-install-recommends ubuntu-mate-core
ubuntu-mate-desktop
