Adobe Illustrator cs3 registering issue

ok, i bought a new , sealed version with correct serial key and went to install.  I know its an older version no longer supported so i downloaded the .dmg file that i found researching here in the forum.  I got the "new key" like it says to do, i downloaded the .dmg and everything it said in instructions.  I followed step by step what instructions say to do and it still does not register the key so im on trial bases essentially and at 30 days im done.  How can i correct this issue? thanks in advance