A: It is not compatible with OSX Yosemite.
Native Instruments made some updates to their effects plugins in the past year. NI recently released several statements addressing Komplete Players 1-7's compatibility with OSX Yosemite. In the conclusive statement addressing this issue, NI states "KOMPLETE 7 and earlier versions of KOMPLETE (including versions of KOMPLETE ELEMENTS) are not supported on Mac OS 10.10 Yosemite (and forthcoming versions of OS X)".
KOMPLETE 1-7 Installer Error Message: "Failure. Installation cannot proceed [...]" (Mac)
Mac OS X (10.10) Yosemite Compatibility News
On January 17, 2014 PreSonus released the following statement:
*Current PreSonus customers who have already registered Studio One and may be upgrading, may already have a Native Instruments key for Komplete Players 7 (Artist) or Komplete Elements 7 (Producer and Pro). If you would like to upgrade your version of Komplete Players/Elements, you will need to install the latest version of the Players found at: http://www.native-instruments.com/en/products/komplete/bundles/komplete-players/ This will upgrade your version 7 software to version 8.
NOTE: If you have not created an account with Native Instruments, please do so now.
Creating a Native Instruments Account
• Go to: http://www.native-instruments.com
• Click on the login link on the upper right hand corner of the site and choose "Create Account".
• Enter your credentials and hit Create.
• Look in your email for your username and password.
• You will need the username and password for the NI Service Center activation.
Additional information on installing and or upgrading to Komplete 8 can be found in this Knowledge Base Article:
 Installing 3rd Party Content (Native Instruments Komplete Players/Essential) (New Bundles)
For more information on the complete registration, activation, and installation of Studio One and its content: