i want to download a purchased song onto my iTunes but i don't know how. When I am in the store, they buy button is a greyed out box that says purchased. Please help!

Hello there, dancebaby.

It sounds like you are wanting to download previously purchased content to your MacBook. The following Knowledge Base article provides the steps for downloading your previous purchases onto your computer:

Download past purchases

http://support.apple.com/kb/ht2519

After verifying that your content is available for redownload per the instructions:

Download previous purchases on Mac or PC

Make sure your Mac or PC meets these system requirements:

• iTunes 10.3 or later
  You need iTunes 10.6 or later to download previously purchased movies and TV shows.
• iTunes account in a country where iTunes in the Cloud is available

iTunes Store

1. Open iTunes and go to the iTunes Store.
2. Click Purchased in the Quick Links section.
   
3. Select the content type that you want to download (Music, Movies, TV Shows, Apps, or Books).
4. Select Not in My Library to view purchased content that isn't on your computer.
   
5. Find the item that you want to download. You can browse using the menu to the left. If you have more than 2,000 songs, you will see Recent Purchases instead of All Songs or Albums.
   
6. Click the Download icon next to the item you want to download.
   If an item is already in your library, the Download icon won't appear.

Thanks for reaching out to Apple Support Communities.

Cheers,

Pedro.