Structs with attribute isDcpsDataType set to true are candidate types to use for Topic types. An example Struct is shown below.
A Struct is created by selecting a Struct from the palette and adding to the diagram.
After giving your Struct a name you will then want to add Fields to it as discussed in Types with Fields.
After you define the Fields for the Struct you may want to re-order them. You can do so by dragging a field and dropping it into the new position.
As described in the OpenDDS Developer's Guide available at the OpenDDS download page, keys are fields used to identify different instances within a topic. These key fields are identified by elements in the Struct's Keys compartment whose names are that of the fields that act as keys.
Keys for the Struct are created similar to how Fields are created. Select the Key tool from the palette and add it to the Struct. Then in the Struct figure type the name of the Field that should be used for the Key. Similar to how Fields can be re-ordered, Keys can be re-ordered by dragging and dropping.