Dear Alvaro,
Indeed, we do not have a factory for that Lie group.
To add support for SL(n) to Manopt, you need to create a factory file. As an example, you can check out the spherefactory, and read more about how factories work on the tutorial page:
There, you can find a table explaining the different functions that need to be implemented to manipulate points and tangent vectors, compute inner products, retractions, projections to tangent spaces, etc.
You can also read more about these in my book:
Don't hesitate to come back with questions about how to put this all together.
Best,
Nicolas