InDesign won't support mathml.
You can use external editor like mathtype for this workflow.
Other plugins are powermath, inmath etc., but it won't support mathml (I think so...), those plugins have it's own codings.
Vandy88 is correct -- InDesign doesn't support MathML, but MathType does, and you can use MathType with MathML. However, I don't want to cloud the issue or introduce confusion -- using MathType with InDesign means you'll insert the equations into the document as EPS, not as MathML. Even if you use MathType to process the MathML, the MathML equations still aren't recognizable by InDesign, and they'll need to be in graphical format.
Unless you're tied to InDesign, there are some really nice XML+MathML solutions available in the marketplace, but this probably isn't the right forum to discuss that.