The version 3.1 of 24U Plug-In Template we have on our website can be used to build 64-bit plug-ins, but it's designed to be used with the FileMaker SDK and development environments that were current in the end of the year 2012.
Since then everything has been updated, including FileMaker SDK, XCode, Visual Studio, and system libraries.
If you use the old SDK and old development enviroment, you will most likely be able to build a functional 64-bit plug-in.
But if you want to use the most recent SDK (which is recommended by FileMaker) and the most recent development environment (which is recommended and may be required in order to be able to use the newer SDK), you have to adjust the project settings to reflect all the differences between the old SDK and dev environment and the new.
Unfortunately, because the number of changes made in both the SDK and the dev environments since 2012, such as type definition changes, has not been small, the amount of work necessary to update the template accordingly was significant as well.
So we decided to stop providing free updates and support for the template.
We can, however, still help you with your plug-in as part of our paid services.
You can learn more about our service targeted specifically at taking care of FileMaker plug-ins at http://www.24usoftware.com/64bit-plugin/
More information about our paid support in general is available at http://www.24uSoftware.com/DevSupport/