The Item command is used in Custom..EndCustm and ToolBar..EndTBar blocks (both of which create toolbar menus) to add an option to one of the tabs. See these commands for more details on how to use it.
Inside a Custom..EndCustm menu, the correct syntax is Item text (text being a string). This will display text for the menu option, and also paste text every time the option is selected.
Inside a ToolBar..EndTBar menu, the correct syntax is Item text,label. This will, as in the previous case, display text for the menu option; when the option is selected, the program will resume running from Lbl label.
130 - Argument must be a string happens when the option text isn't a string.
500 - Invalid label happens when the label doesn't exist (in this program), when the option is selected.