The best place to look for help/sample code is the developers forum: Eclipse Community Forums » Model Driven Health Tools.
You may need to create a (free) account to get access.
You could also go to the forums to post your specific MedicationsSection/immunization question for a possible specific answer.
Another good site is CDA tools: MDHT Developers Guide. Look at Produce CDA Content using MDHT API.
There are countless examples of building documents in the test code projects.
Download the All In One (MDHT_CDATools) from GitHub linked in the MDHT project site and look at the code in the test projects such as: org.openhealthtools.mdht.cda.consol.example
The closest (current) thing to a GUI based document building application in MDHT would be using the all in one to modify the existing models and generate sample snippets/documents that way (or creating a model from scratch, which includes only what you need). If you wanted the entire document produced in XML, you could then generate the instance from GeneralHeaderConstraints as opposed to any of the many child templates (which would give you snippets). Either way, this is not really the intention of the model interface (it's more for conformance) and would take you far longer to implement than using the API itself - which accesses the models which already exist in order to auto-generate conformant content quickly.