To set a default application in OS X (for instance, if a client has installed OpenOffice and wants .doc files to open in Microsoft Word instead of in OpenOffice),
- In your Finder, select any file of the type you're trying to change.
- Go to File > Get Info.
- If it's not already open, expand the Open with: section towards the bottom of the resulting window (beneath Name & Extension, above Preview).
- Choose the desired application from the drop-down menu (it should appear in the list; if it doesn't, click the Other... option and locate the desired application within your Applications folder).
- To make this change default for all documents of that type on your computer, click Change All....
To change Apple Mail to be the default program for sending messages when clicking mailto: links in Safari, launch Mail and go to Mail > Preferences. Under the General tab, set Mail as Default Email Reader.