I've managed to get the problem of adding a custom SSIS component to a Visual Studio 2015 SSIS package. This maybe have been enabled by the release of SQL Server 2016 CTP 3.2, but I thought that I'd put this out there in case it helps anyone.
First of all let me just let everyone know what system I'm using in case this doesn't work for everyone:
My system is: Windows 8.1 64 bit
Visual Studio Community 2015 Version 14.0.24720.00 Update 1
Microsoft SQL Server Integration Services Designer Version 13.0.900.80 (I've installed the SQL Server 2016 CTP3.2 so the version of SSIS is this version - this may be part of the problem and it isn't a fully released product and this may be a bug)
Microsoft .NET Framework Version 4.6.01055
In my c# class project I used the .NET Framework 4.6
(Project Properties->Application).
I made sure that I added to the gac using the gacutil.exe in the following folder:
C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6 Tools
and I copied the dll to the following folder:
C:\Program Files (x86)\Microsoft SQL Server\130\DTS\PipelineComponents
After refreshing the SSIS toolbox the component now appears in the Common folder/subsection.
I hope this works for everyone or is at least of use to anyone else who has had this problem. I was so happy after trying nearly everything and it coming up blank to see the component appear in the SSIS toolbox and know how frustrating this kind of thing can be!!!
Matt