go to the Modifiers tab
click New Modifier
click on your new modifier (left pane), set mod type to Macro (right pane)
click Assign Mod, press the button on your controller for which you want turbo, and check the appropriate box in the right pane (the N64 button that will be turbo)
click Rapid Fire Mode, click Apply Changes
go to the main configuration tab (Controls) and unassign the button for which you are making turbo (click the configuration button, and then press Escape
If you want to disable turbo, you need to go back to your modifier, uncheck Rapid Fire Mode and click Apply Changes.