Trouble Downloading Lightroom 3.6 on my Mac

Can someone please help me with this?

In the past I purchased the software online, so I have my receipt in an email with the serial number for the software etc.  I originally had downloaded the windows version on our dell laptop, and I remember running into this issue where it would not open lightroom right away, and it kept asking me for the id number or whatever.  Drove me crazy but I didn't have much time to play around with it so I quit trying all together.  The laptop screen is now broken so we no longer use it.  I have since purchased a brand new imac and I remember having the option to download a mac version as well as a windows version so...today I got online and downloaded Lightroom 3.6 from my history, its on my desktop but when I try to open all the window shows is the pkg file & pdf file.  If I click on the pkg file, it just asks to be installed.  I am still learning how to get around my mac too so this is driving me crazy.  HELP!  oh and is it possible to download Lightroom to another windows pc now that my laptop won't be used or have I lost that download?