So far not every VSM screen could be customized within Designer. Some of them can be changed by changing template file (*.htm).
* So if you wish to change the KB entry details screen for edit mode use Designer (Screen Design: Service Desk; Screen Set: any from the list depending on your KB entry type).
* For KB entry review mode screen you have to change files InKnowledgeReview_[KB_ENTRY_TYPE].htm directly. Don't forget to recycle VSM IIS app pool after changing these files.
By the way, I have a strange feeling that you should be able to change these review screens with Designer. I guess (just my blind guess) it will be fixed or implemented in the nearest future.