Hi, I wish to create add ins for Microsoft Dynamics NAV/ Navision. Is there any API / SDK that I can use to create add-ins?
Christian Abeln is a SPM for Dyanmics NAV at MSFT and his blog has some good resources for introductions to add-ins in NAV 2009 SP1.
Add-ins require an interface (which tells RTC how to communicate) and then a class that implements the interface - these will live in a managed assembly DLL you can create in Visual Studio.
What type of add-in are you looking to build?
Stackoverflow is a great resource, regarding general software development questions. Dynamics NAV is rather a niche product, here you will hardly find lots of info, concerning development for it.
I think that all (or most) information concerning specific Dynamics NAV tools (there are not so many) can be found (and should be looked for, in the first place) on the Microsoft Dynamics PartnerSource site. If you have no access to it (it restricted to partners only), then I can recommend www.mibuso.com forum - one of the biggest resource of Dynamics NAV information.
If you are a member of Dynamics NAV partner team and have a NAV development license (which gives you access to NAV development tools), then all that is said by me is obvious for you and here you could find a help for the rest of your needs and Visual Studio is luckily the main needed tool.
If you only need to manipulate data you should use the CFront SDK. This provides programmatic access to the database in Nav 5, but CFront does not run any of the business logic.
If you need to extend the client you can probably use COM. A good place to ask would be on the Mibuso forums.