First of all, set the project Properties / Audio tab to 5.1 in the Master Bus Mode field.

Then in the header of each audio track, set the L/R/C/RL/RR in the Surround Panner appropriately for the audio media on each track:

For stereo, only the L + R points in the Surround Panner should be active unless you want the stereo audio to be affect all channels (a la 5.1) in which case all five channels can be left active.

EDIT: From the screenshots you've uploaded, it does not seem that Properties/Audio has been set to 5.1 because there is no Surround Panner in the track hesders. Based on that, you are currently working on a stereo project no matter what other settings you are making.

Your screenshot shows a separate track for L/R/C/RL/RR/LFE so once Properites/Audio has been set to 5.1, the Surround Panner in the track header will need to have only the approriate speaker node activated for the audio event on that track: i.e. only the L speaker node active for the track with the L speaker audio event, and so on. You can turn on/off the speaker nodes by simply clicking on the node/s.

For the LFE track, R click the Surround Panner and aelect 'LFE Only' from the context menu that opens.

Something like this: