Hello
The command to load descartes if you have a Microstation session already open is MDL load dcartes.ma. Is there a way to create a macro or button that you can place on your Microstation screen, for people to press if they wish to open Descartes?
In addition to Pascal's suggestion, you could add the keyins to a pair of function keys.
To do that, go to Workspace > Function Keys.
Select a key and add mdl load dscartes as the action on one key and mdl unload dscartes as the action on another.
Save the changes.
For addtional information search Help for "Function Keys".
And you'll also find some stuff in this article:
http://communities.bentley.com/Communities/Other_Communities/AskInga/w/AskInga/all-about-function-keys-and-function-key-menus.aspx
~Inga