from the repo you pick the package in Synaptic, then click apply.
to install one you downloaded that isn't specifically compiled for elive:
set up a local repository and put it in there.
point Synaptic to the local repository and install it.
[ select the app and click apply ]
the complexity of having to make a local repository is the biggest drawback of the apt-get backend for Synaptic.
the rpm based distros you do not need to make a local repository, you only need the root password to install an rpm from the folder you downloaded it to. much easier.