When you clicked "specify", what options did you select in the dialog that then opened?
I tried both Calculation and Script Text.
For text, I used the text "open document/application; "cmd.exe /C MKDIR " & Quote ( $desktopUse & $dirName & "\\" )".
For the calculation, I entered the field name where I had the text
"open document/application; \"cmd.exe /C MKDIR \" & Quote ( $desktopUse & $dirName & "\\" )"]
both with and without the delimiters before the quotes in the "cmd.exe..." part.
You need to use the calculation option, not text.
You would not enter a field name, but enter "cmd.exe /C MKDIR " & Quote ( $desktopUse & $dirName & "\\" ) as the calculation after selecting open document/application from the drop down menu:
Thanks, Phil. I still had a little trouble until I realized that I was building this xplat solution on a Mac (the dialogs were different). Once I opened the file in Windows, things looked the same as in your screen shots, and I could create the script step properly.