Onotoa is an Eclipse-based ontology editor for Topic Maps that provides a graphical UML-like interface, an export function for the current TMCL-draft and a XTM export.
Instead of editing a text file, Onotoa allows you to create the schema in a visual editor. The visualisation is based on the GTM level 1 standard (see http://isotopicmaps.org/gtm/).
Additionally, Onotoa makes it possible to create schemas visually. As the old saying goes: "a picture says more than thousand words" - a diagram which is easy to understand is clearer than hundreds ofCTM text lines.
Moreover, you will be able to export the schema to CTM. So if you want to have the CTM using the TMCL templates, you can create them.
Also you can import a CTM schema, so you can re-use schemas which had been designed previously and then create some diagrams.
Detailed instructions on how to install and use the Onotoa utility on your Mac are available HERE.
Onotoa is cross-platform and it works on Mac OS X, Windows and Linux. Binaries for the Windows and Linux platforms are available on the project's homepage.