Event calendar plugin that allows you to add a nice-looking scheduler/planner with drag-n-drop interface, recurring events, Google Map integration.
The scheduler was tested with most popular themes for WordPress, but still it’s possible that theme used in your case is not compatible with the scheduler's styles. Please drop an email to dhtmlx [at] gmail [dot] com with the name of used theme.
Most probably you are using php 4.x , which is not supported. In settings of plugin enable "Debug mode" and check the problematic page again - now it must contain more readable problem description.
All operations can be done through the public GUI documentation
Also, you can create new events during post creating | editing documentation
Check the settings of scheduler, the "Add" action must be enabled for the related user group, to be able to add the new event.
If something is still not clear - you can ask your question at dhtmlx support forum