First, you need to check the Cubase 5 manual Chapter 12 ‘VST Instruments and Instrument tracks’. Specifically ‘The VST Instruments window’ section page 171-172. This will setup Cubase for multi output channels for the VST instrument (in this case EZDrummer).

“The rightmost button allows you to activate the desired output for the instrument. This is useful when you are using VST Instruments that have a large number of audio busses, which may be confusing. Click entries in the pop-up list to activate/deactivate output busses for this instrument.”

In addition, check the manual for VST Instrument channels vs. instrument tracks (page 170) and make sure you’re loading EZD as an Instrument channel (loading EZD in the Instrument rack; creating a MIDI track; routing MIDI Out to EZD; enabling the track monitor button) and not an instrument track (selecting  ‘Add Instrument Track’ from the Project menu). Instrument track are stereo only and will have no multi channel options.