Hi all!
New here 
My problem is that after installing Unity I want to develop for the new Flash platform and I cannot select this option when I create a new project.
Do I need to import some package?
No you need to install the developer preview.
The stable build (3.4) does not support flash.
Also be aware that Flash will not be a free platform when it really lands, only the developer preview (prebeta kind of) is free
Thank you!