1. OK. First off, when getting 3rd party instruments or effects for Logic, make sure you get the Audio Unit (AU) version as you've mentioned.
2. Quit Logic if it is open.
3. Drag the audio unit (should be a .component file or folder) to the following location on your Mac (where your user folder is represented by ~ )
4. Open Logic. It should take a little longer than usual as Logic will verify that the AU Plug-in is stable for use.
5. If you have downloaded an Effect plug-in you will be able to find it by clicking on an Insert slot on any audio or soft. inst Channel and scrolling down to the Audio Units part of the drop down menu. You will find it there listed by Company name.
It's the same procedure for Instruments, except these can only be accessed on soft. inst tracks by clicking on the I/O slot.
Most commercial AU plug-ins will automatically install themselves to the correct location. But for free AU's the above can be very useful.
Also, useful to know where your Audio Units are kept in case you need to remove them for whatever reason.
Lastly, Bj, you'll find that Turntablist is actually an Instrument NOT as Effect plug-in! So, you'll need to create a soft Inst track and open it appropriately.