Looking for Muted Trumpet

In the same place you would load any other virtual instrument. Create a new Instrument track, on the left (see graphic) the instrument slot is right above the I/O label. Click and hold on the slot and you will see a pop-up of available instruments, scroll down to AU instruments/Apple it should be there somewhere.. I'm on Logic 9 the graphic will look a little different but everything should eventually be in the same places.

Cup, Harmon, Straight?

Depending on how desperate you are....😀

You can try the Apple DLSMusic Device which has a generic General MIDI set, there's a muted (Harmon) trumpet at patch 59 (counting 00 as patch 1). Load the DLS Music device into an instrument slot like any other virtual instrument.

Would probably sound ok in an ensemble.

If you're still interested (sorry, I don't look at the forum too often lately) this is a nice Harmon muted trumpet and there's also a really nice Flugelhorn there at Wavesfactory as well - you need kontakt for them, though really but you have access to the samples (it wouldn't be that straightforward to put them into another sampler though, unless you're used to that kind of thing) -

http://www.wavesfactory.com/mutedtrumpet-harmon