Import existing data in Matrix fields We have an existing CMS and I'm just learning how to use CraftCMS so that I port the data from SQL Server to MySQL database. I went through a few videos on how to create a basic site so I kinda understand the system.

I have used a couple of plugins to import the data and one of them being this:
https://github.com/boboldehampsink/import 

This plugin doesn't export existing data from MySQL data from Matrix field (using sample data). I wanted to see how the data is stored so I can create a similar format. But unfortunately the Matrix field comes back empty. I am not sure how to create and insert data in CraftCMS for this specific field. I will also at some point need to retrieve this data so I would like to know how the insert/select works on these fields.

Thanks in advance.
I'm not 100% clear on exactly what you're looking for, but I'll just throw this out there.  Matrix content is saved in their own tables (table names are dynamically generated on a per-field basis) as opposed to most other fields, which have their content saved in the craft_content table.

You can look in MatrixService for methods needed for managing Matrix fields.