The properties for an object can be viewed (and modified) within the Object Inspector window. The inspector can be toggled on/off using CTRL-ALT-O, and used as either a "Floating window" or docked on the left or right hand side of the screen. If multiple objects of the same type are selected, it will show the properties of the first object in that selection. When altering values in the inspector, the properties of all selected objects are updated.
Sometimes you want to add some user comments to an object, as reminder to yourself or others working on the same project. To make editing of comments more convenient, press the little (...) that becomes visible when you edit comments field in the Object Inspector: the Comments Editor will pop up, allowing you to edit your text in a nice 80-column monospace font environment.
DeleD is a generic modeler, and has to be able to export to a lot of other applications. Because it would be undoable to natively support every possible characteristic featured by those applications, every primitive, light and material in DeleD supports the concept of custom properties. It's up to the exporter/reader to interpret this data in a way that is appropriate for that specific application.
In it's simplest form, you can simply type some information in the Tag property. For more advanced property editing, hit the (...) button at the right hand side of the Tag field, and the Custom Property Editor will appear. This editor allows you to define arbitrary key-value pairs. Use the arrow keys to navigate, ins to insert a new value, and esc to revert the latest modification. The custom properties will be stored inside the Tag field, separated by a pipe symbol ('|').