I can't think of a scenario in which you'd be able to view a Mashup for a edit, but not add new widgets. Are you running the latest version of ThingWorx (4.2)? Is there anything in the logs that suggests an error is occurring? What is the environment in which you are running and working with ThingWorx (i.e. operating system, web browser, etc.)?
Please be as detailed as possible when providing your environment information. Keep in mind, ThingWorx only supports 64-bit operating systems and your version of Java and Apache Tomcat will be useful as well.