Before entering a User Symbol, see How to create a user symbol.
To enter User Symbol number 6, for example, move the edit cursor to where you want the User Symbol to appear and type A then 6. To enter User Symbol number 16, move the edit cursor to where you want the User Symbol to appear and type A then SHIFT+6. User Symbol number 16 will appear.
You can also enter a user symbol by selecting the user symbol you want from the User Symbols symbol palette and entering the symbol with the pencil tool.