You can start filling the form from the scratch (but Dashboard tries to remember as many values as possible from your last visit), or you can check the box Fill...when selected, and then select an existing entity in a tree. It will copy selected entity to the form fields so you can use it as a template (if you forget to change the name of the entity, registry refuses to register it, because such entity already exists there). All entities, except services, have the tree for selection directly in the registration panel. For services, you need to select a service in the Registry Browser panel.
In order to unregister, use either a Unregister... button (for services), or select entity you wish to unregister, RIGHT-CLICK in the select tree, and click on unregister (for other entity types).
The Biomoby registry does not have direct ability to change entities once registered. You need to un-register them first, make a change, and register them again. Because Dashboard remembers things, it allows to do changes more easily: Select an entity (let selection fill the form fields), un-register it, make changes, and register it again.
The new data type is built on the left side, in a new tree, showing just the new data type. You can again remove children by RIGHT-CLICK. The same way you can (and you should) change the article name for every child. The display is still somehow buggy, try refresh, or collapse and expand the tree if it is not displayed correctly.
The service inputs and outputs are defined on the right. Add them by clicking on Add buttons. You can always remove them again by clicking a trash icon in the table. All input/output data should have an article name.
Each primary input and output has to have a data type. First select a line in the table (a line is already selected when you hit the Add button), then select a data type in a data type tree below the table. Then consider to change the automatically filled article name to something more understandable. For each data type you can add one or more namespaces from the other tree there. The added namespace can be again removed by holding CTRL and selecting them in a pop-down menu.
The secondary parameters are not too user friendly (should I improve it?). Especially the column allowed requires a comma-separated list of allowed values (if there are more values available).
There is also a button Call RDF Agent. It can be called only if the RDF endpoint is enabled and not empty. Then, it calls the current BioMoby registry with (and only with - the other text fields are ignored) an RDF signature URL - meaning that a registry agent is going to retrieve a document defined by this RDF URL and to synchronize its contents with the current registry (the document pointed by this RDF URL contains your services definitions).