This error has the following causes and solutions:
If you don't indicate that an add-in is command-line safe (even if it is command-line safe), when a user selects your add-in and then Command Line in the Load Behavior box, they'll receive the warning message. This isn't a serious problem, but merely a warning to the user that the selected add-in might possibly contain UI elements that can pop up unexpectedly and halt their automated scripts by pausing for user input.
To specify that an add-in is command-line safe, author it that way (you can use the Add-In designer for this), or manually change the value in the registry key.
For additional information, select the item in question and press F1 (in Windows) or HELP (on the Macintosh).