You'll need to set the "root" instruments folder. To do this, click on "Prefs" in Philharmonik to open the preferences window, then click on "browse" to select the proper location.

By default the Miroslav Philharmonik Instruments folder should be located in the Macintosh HD > Library > Application Support > IK Multimedia > Miroslav Philharmonik folder. Be sure to select "Istruments" within this location and then click OPEN.

NOTE, if you've moved this Instruments folder to another location, you'll need to go to "Prefs" and browse once again, selecting the new location.