I want to write and addin for WealthLab Pro and after reading through the documentation, I could not find any on addin programming. The addin I want to write would be extending the functionality of WealthLab in a way similar to NeuroLab addin extension.

Specifically, I want functionality similar to MarketAnalyzer(in NinjaTrader) or RadarScreen(in TradeStation). Or if there is any other way to add custom fields in Quote window, that would work as well. The intention is to create an addin and open source it so that people can modify and bug fix it.

Any pointers?

RadarScreen and Neuro-Lab serve two strikingly different purposes, but I see what you're after. There are lots of APIs in Wealth-Lab 6 to extend almost any aspect of the application, from adding new chart drawing objects to inserting a broker provider. Most of the API guides can be found on the dedicated Wiki pages:

Wealth-Lab Version 6 (.NET) Development Guide
Pages of the Category API

It's not possible to extend a Quotes window but you could try build your own "mini app" and stick it under the Tools menu as explained here:

Wealth-Lab 6.x API Changes > "Inserting custom items under Wealth-Lab's Tools menu"

or insert a custom Data Manager tab as documented here: API | Creating Static Data Providers for Wealth-Lab 6 > "Creating a Data Manager tab for your data provider"

Both links are available after WLP/D entitlement verification (open a new support ticket).