I had this same issue with the MotoPress Content Editor Lite v.1.4.8 and found a solution:
Make sure you have activated the plug-in under the MotoPress -> License tab in your WordPress Dashboard. You get a 6-month free trial. After activating the plug-in, the update for v1.5.8 will be available via your WordPress Dashboard. Update as you would any other WordPress plug-in and problem solved.
Thank You
