You can install from the Sid repo by adding it to your sources.list. You need to comment it our when not using it or you may get a surprise upgrade.
You will of coarse need to run "apt-get update" or better in my mind for this particular job "aptitude update".
To install a package from sid you would then, in apt-get, run;
apt-get -t sid install <package name>
aptitude install <package name> -t sid
Aptitude really does have excellent dependency options when problems turn up.
If you keep putting alien packages on a testing install you WILL have problems. If you can't find it in the testing repo check what is available in sid or even experimental and use them instead of any 3rd party package.
If you insist on some alien package learn to build a .deb out of it and use dpkg to install it.