As you know Intellij Idea comes in tar.gz distribution for linux OS, so it is only possible to launch it from terminal after unpacking.
But there is a possibility to add quick launcher button for unity.
First of all, create idea.desktop file with the following contents:
Name=Idea Community Edition
Comment=Launch Idea Community Edition
Paste your location to the Idea distribution in the italicized text given above.
After that, open this file in nautilus and just drag it to your launcher bar.