To install synapse in Ubuntu 14.04, open terminal and type

sudo add-apt-repository ppa:synapse-core/ppa 
sudo apt-get update
sudo apt-get install synapse
It should install synapse in your system.

synapse is now available in Ubuntu Universe repository (See here). To install it you need to enable the universe repository . Enable it using

sudo add-apt-repository universe
sudo apt-get update
sudo apt-get install synapse