Instead of this command I recommend using drag & drop functionality. The command is available when you select several files in the list with results and then right click them and choose Open With ...
Here you specify the program you want the files to be opened with and additional arguments in the textbox labeled Arguments: The '&files' will later be replaced by the sequence of the files that have been selected (ex. &files = " "document1.doc" "document2.doc"").
You cannot open more than 50 or so files with this command. Use drag and drop instead.