Title says it all. How do I start?
I thought I can get help from Linn (oss.linn.co.uk), but I can not find any link to their source code.
If you want to use Linn's code you'll have to register as a developer with them. See the bottom of the Developer Programme page for details.
Recent and all future Linn UI products will replace their OSS UPnP stack with ohNet however so you'd be better using that. ohNet is available as open source and is liberally licensed so suitable for inclusion in open or closed, commercial projects. It is written in C++ with the option of C bindings for its public APIs so will be much easier to deploy on Mac/iOS than the OSS stack which is written in C#. This would however leave you needing to port their C# app to Objective C...
Disclaimer: I contribute to ohNet so may not be considered entirely unbiased.