Struct and Union have Fields which are named references to types (similar to class attributes). To add a Field, click on the Field tool in the palette and then click on the Struct or Union to contain the Field. You then fill out the Field using the form "name: type". For example, to indicate that a Field is to be named count and that points to a Long you would enter "count: Long". (Basic types do not need to be created first.) The type can be from any of the active DataLibs in addition to the types in the currently opened diagram.
If a Field has not been defined properly (for example, a reference is made to a type that does not exist) you will see <...> in its place. Either edit the field using the "name: type" form or delete the field from the Struct.