Audio mixdown on cubase artist 6

Hi, I have just got started in cubase artist 6… I have used a midi keyboard to trigger HALion sounds for my first musical experiment…I can play back the tracks in stereo but unable to mixdown in stereo…above each channel MIXER(F3) is either Mono 1 or Mono 2 (which gives the stereo sound)…On File>Export>Audio Mixdown the Output Channels show Mono 1 and Mono 2 tick boxes only on the left side of the box, so when I export it is a single mono mix of all intruments…Problem: I am unable to select the STEREO out tick box on the Output Channels following File>Export>Audio Mixdown…where have I gone wrong?

Thanks in advance,