If it isn't already, right-click and select 'Set as Default Device.
In the 'Playback' and 'Recording' tabs, make sure the Focusrite option is selected here as the default device.
Step 1 - Before opening Mixcraft, go to your computer's Control Panel > Sound option. If you have Scarlett 1st/2nd/3rd Gen, Saffire or Clarett interface, the steps will be exactly the same however the driver shown in step 2 (ASIO Device) may appear slightly differently for Saffire's & Clarett Thunderbolt. This article shows you how to record audio into Mixcraft using a 2nd Gen Scarlett Solo audio interface.