Mixlr AudioLink is an additional audio device which is included when you install Mixlr. Because of the nature of the device, if you're using a Mac you will need to approve the installation of Mixlr AudioLink when you first want to use it, and it won't appear as an option in other applications until you have done this.
To approve the installation, select Mixlr AudioLink as an input in the Mixlr app and then follow the prompts, and you should be able to approve it and start to use it successfully.
Still unable to use Mixlr AudioLink?
If you're still unable to use Mixlr AudioLink after following the steps above, it's possible that this is being preventing due to your system settings. If that's the case then you may need to change these settings via the Terminal in order to allow the installation of Mixlr AudioLink.
The article below explain how you can access the Terminal on your Mac so you can change the settings to enable you to install any programs.
Once these settings have been updated you should be able to install Mixlr AudioLink via the Mixlr application as you have tried to to do earlier (you might also need to restart your machine).