I know how. Go into Finder and press Command+Shift+G. Copy this (~/library) without the parentheses and click "go". Locate the Application Support folder and locate the minecraft folder inside it. Look for the folder labeled "saves" and click it. Right-click the map you want to be your adventure map and click on "Compress (Then the world name, it will be the original world name btw)". Then locate a site such as mediafire.com and create an account. Go into your files and click the "Upload button" Locate the .zip file and double click it. Wait for it to upload to mediafire.com and close the uploader. Click on the file you just uploaded and copy the link and put it into the World Download URL when you upload it.

Remove the First few steps, click and drag the minecraft folder over to the gray bar on the left, and that folder will stay updated so you don't have to go to the library.

I hope this helped, and if you need any more Mac help, just let me know!