From within Logic, you first need to create a new software instrument track.
As soon as the software instrument track is created, click on the window representing the new track you just created. Next, upon looking in the Inspector, you will find a button that says "Instrument" -- click on that button.
Upon clicking on that Instrument button in the Inspector, the screen will reveal a large number of available software instruments; as you look down towards the bottom of that list, you will find a line that says AU (for Audio Units -- not VST) instruments.
Hover the pointer over that AU Instruments window, and then look for Modartt (assuming you have loaded that AU version of Modartt Pianoteq into your computer); next, hover the pointer over Pianoteq (5), which then reveals a choice of mono, stereo, etc. Click on your choice, such as stereo.
If done correctly, you may expect the Pianoteq software to work, and you are in business.
Hope this helps,
Cheers,
Joe