this video shows you how to get a texture pack on Mac OSX Lion.
it has all the stuff in the video but i will tell you here because there is a word minimum which is 350:
1. go to the page you texture pack is on
2. go to the download page and download it
3. you find it in your downloads and un-zip it using a unzipping application
4. go inside the un-zipped folder and select all the content
5. right click and compress items
6. drag it over to your texture pack folder
7. open minecraft, click on texture packs and it should be there