You need to update the dependencies to point to Studio 2014. You can do it in the same way as if you would update a plugin from 2014 to 2015. More about that here:

You can download the Studio 2014 SDK from the developer page:

First of all you don't need the SDK to make your plugin work with Studio 2014 but you do need Studio 2014 installed in order to build and test your plugin. The reason I shared the video is because you can use the same approach to change the path from Studio 2015 to Studio 2014 dll's. What you have to do is in the opposite direction as what is shown in the video. The path of the assemblies is based on the install location of Studio 2014.

Hi Jeroen,

I'm not a developer, but it sounds to me as though you have not done what Romulus suggested.  You must have Studio 2014 installed if you want to use the Studio 2014 SDK.  If you do that you will have the Studio 3 folder.

But probably also worth noting that you also don't need the SDK anyway if you want to create a plugin for the 2014 version (or any version for that matter).  This article that Romulus also published might be a good clarification for you:

Regards

Paul