Hello, vou get this error message cant find you Metamodel and therefore cannot verify that there is a MetaClass "mathset::Model". to tell Xpand where to find / search your MetaModel you have to do only two things
(1) tell xpand what kind of metamodel you are using. In Your case this would be EMF Metamodels (In your projects Properties go to the Xtend/Xpand section and verify if EMF MetaModels is selected (should be default)
(2) the (generated) .ecore file has to be on the classpath of your xpand project. therefor add a plugin-dependeny to your generator projects manifest file.
if you cannot see emf metsmodels in the configuration your xpand installation seems to be incomplete. to you use a distribution as you can get it e.g. from here. the ecore file is generated by the xtext worklow e.g. to /org.xtext.mathset/src-gen/org/xtext/Mathset.ecore