To use the Solar Panel Tool you will need a 3D model with solar panels specifically called out in the 3D model.
Attached is a simple 1U CubeSat model with comments explaining how the model is made. The model is in MDL format, which is STK's native model format.
The solar cells are sized to have the standard power generation of 2.4W per side.
For the model documentation see: http://help.agi.com/stk/#vo/models.htm
For more details on creating solar panels see: https://agiweb.secure.force.com/faqs/articles/Keyword/How-do-I-set-up-a-model-file-containing-solar-panel-information-to-allow-me-to-generate-power-reports?retURL=%2Ffaqs%2Fapex%2Ffaq&popup=true
For more details on converting a CAD model for use into STK see: https://agiweb.secure.force.com/faqs/articles/Keyword/How-to-convert-CAD-models-for-use-in-STK?retURL=%2Ffaqs%2Fapex%2Ffaq&popup=true