You need to add a reference to Mono.Android.Export.Dll when you use ExportAttribute or ExportFieldAttribute

I found the answer. Right click references and search Export. Just add the Mono.Android.Export to your project.

I added reference like @Jeremy Said, it's working

You don't need download dll from the web if you cant find it in the reference manager

just do following steps:

1. Set the target framework to Android 4.4 kit kat
2. Add reference mono.android.expert.dll
3. set targetFramework as last good target framework

Clean solution and rebuild

I hope it helps you.