I assume you have created the code.

You will need to compile it in the same version as the designer you are using.
You will also need to place the dll in qt creater's bin/designer directory.

Once it is in the correct directory open the designer and a form ui. Then: Tools -> Form Editor -> About Qt Designer plugins...
In the popup window you will see if it loaded or if there was an error. And the error should explain what you need to do.

I think for windows designer you get as the standard, you need to compile it with msvc compiler in release... But check the error to make sure.