Go to the View menu, go to Toolbars, and click on Customize… Click and drag the Redirect button to wherever you want the button to be in the toolbar (e.g. next to the Forward button). Click OK to close the Customize Toolbar window.
NOTE: This will add the button to the mailbox list toolbar only. If you want the button to appear in individual message windows when you open them, you should open a message in a new window and repeat the above process.
To redirect messages, highlight or open the message you would like to redirect. You can either use the keyboard shortcut for redirect (Ctrl + B for Windows, Apple + B for OS X), go to the Message menu and select Redirect, or click on the Redirect button if you set one up in your toolbar. In the window that opens, enter the address(es) you want to redirect the message to and click Redirect.