Please help activating unity PRO

i have downloaded the free unity 2.6 installer and so after installing it and so as I am student who's looking for open source i choose the free unity without reading the difference between the free and pro ..

and now that i realized that this free unity is lacking out for something that i need, I deleted the free unity that is already installed on my computer. After deleting the free i install again the unity using the same installer that i downloaded from the unity3d site hoping that i can choose again from free or pro.

but after installing it again its already a free version and the installer is not asking for if i want the pro version that last for 30days.

i want to use the unity pro version but i cant afford such expensive development tools , so how can i activate a 30days trial for unity pro???

PLEASE HELP!!! (sorry for bad english )

In the top Menu.

Help > enter serial Number

Opens a website where you can activate the pro trial.