The most likely reason why you cannot add or edit your existing devices and activities is that the database is locked for the service to access the data.  This is done whenever the scheduler timer is activated. On the 'Activities' pane check to see if the scheduler timer is activated.  If it is, then you should see a 'Stop timer' button at the bottom of this pane.  Click this button to stop the scheduler timer and release the database lock. To start the timer again, click the 'Start timer' button. 

You may also want to check your settings under menu Options|Setup|Misc.  At the bottom of this pane there are two drop down list boxes which allow you to automatically choose how the timer behaves when you start and exit the manager, which you may find useful.