Do you have Mixcraft properly set up to be used with your audio interface/ sound card?
Basic setup (First 3 minutes of vid):http://www.acoustica.com/mixcraft/v7/help/mixcraft-quick-start-guide.html
Do the following to auto-scan and designate any installed VSTs (like EZ Drummer) with Mixcraft:
Start Mixcraft. Under File-Prefs-Plugins, click on "Edit VST/VSTi Folders". Click "Auto-Scan For VST Directories". This will find any VSTs you have installed and add their locations to Mixcraft's VST list. Be patient, this may take a while. When finished, OK out of the Preferences window to the main MC screen.
Restart Mixcraft. Open a virtual instrument track. Click edit button, play instrument.