I'm working on a C# .NET application that needs to integrate to Accpac 5.6. I have not been able to find any useful documentation on the Accpac API. Where can I find Accpac API documentation?
There isn't a huge amount of documentation unless you sign up for Sage's Development Partner Program. In general you can record a macro of a process that you want to do through your integration (like creating invoices, for example) and then translate the resulting VBA code into the language that you're using.
Joining the Development Partner Program is likely overkill for what you're needing. Typically you'd ask your dealer (is you're a customer) for the documentation. There are a few other sites around that have some information like Tek-Tips.com. If you have specific questions then we can try to answer them here at SO.