You need to copy the dtsx package to following location and restart the SSDT for VS2012. Note that the VS installation location may be different in your machine. First select the package that you want to export as a template and then Select File->Save As.. and save it to
C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\PrivateAssemblies\ProjectItems\DataTransformationProject\DataTransformationItems
E.g. C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\PrivateAssemblies\ProjectItems\DataTransformationProject\DataTransformationItems\MyTemplatePackage.dtsx
Quote: I have also tried to use the Export Template Wizard but this feature seems to have gone away in SSDT for VS2012
SSDT for VS2012 does not have an export template wizard. It's only avaible for VS2012.