how do I scan with a Brother MFC-6890CDW

If it's like all the other Brother MFDs I've used, there should be at least three basic methods:

1 there should be a TWAIN extension which ships with the system as part of the standard software install. That extension should allow Photoshop, Graphic Converter, Acrobat, ReadIRIS, and other apps which use standard scanner extensions to scan.

2 Apple's Image Capture, which ships with the OS, should recognise the MFD.

3 There should be a 'Scan' button and a 'Scanner' button in System Preferences/Print & Fax and in the printer applet which spawns from Print & Fax and whenever you print something with that MFD. Click on either button and you can scan.