We don't support Focusrite's products directly on this forum, however they are a 3rd party hardware OEM partner for Cubase LE7. I would advise you to contact Focusrite's support for more specifics but I can give you an overview on how to setup your interface with Cubase LE7 on OSX 10.9 Mavericks.
First, go to your manufactures website to find the most recent supported CoreAudio driver for your Audio Interface and OSX 10.9 Mavericks.
After Installing the driver, be sure your Audio Interface is connected to the computer via USB.
Launch your Cubase LE7 Application and create or open a project.
To select your Audio Interfaces driver please go to Devices>Device Setup> click VST Audio System and at the top of the window select the Focusrite Scarlett driver to your Audio Interfaces. Please be sure the correct audio driver is selected before closing this window.
Next you must go to VST Connections to setup your Input/Output configuration to your Audio Interface. To find this go to Devices>VST Connections>Check the Output Tab first then click on your Device Ports and Select your Audio Interfaces output options. Next check the Input Tab and click on your Device Ports to Select your Audio Interfaces input options. If the Input or Output Tab options say "Unmapped" then click or double click the box that says unmapped and select the correct buss from the menu below. Next, click the checkbox that says always map and click "OK". If you have more Inputs or Outputs on your audio interface select "Add Buss" to configure additional I/O then exit out of the window.
**Please Note** If you are running other applications such as YouTube, Windows Media Player, iTunes and Cubase at the same time, be sure in Cubase to go to Devices>Device Setup and check the box that says "Release driver when Application is in Background".
**Please Note** Be sure to have the input monitor button selected on tracks you are recording and de-selected for playing back recorded audio.
**Please Note** When you select an external Audio Interface as your input and output device, all sound will go in and out this selected device, meaning your audio will not play back through the computers internal speakers or those connected to it's internal sound card. You must either connect studio speakers to your Audio Interfaces' outputs or connect stereo headphones to the interfaces headphone jack.
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