Thanks Rod.
I found an interesting google project: Web-P
I see that you have talked about webp in forum.

The point is we need an animation format that work with lossy compression to "save for web" (Getting smaller sizes than GIF or apng)

I just looked in that same zip file just to check and there are THREE files in there. do you have your system set to view ALL hidden files?
Windows tends to assume everyone is too stupid to know what is inside their computer.

There are three files in the one I downloaded too.

More interesting would be a apng optimizer program.Something that runs like RIOT, but with animated PNG files only.
Just to get the file sizes down to a bare good web quality minimum.
But can you have your cake and eat it too?

@ Cecilia and Molly - Thanks for confirming the 3 files are in the zip download.

I tell you i have never had any problems loading, starting, or using plug-ins, scripts, and resources while using Parthas builds. I recommend them in cases such as these.
Download his portable and try the apng files in that build. I bet they will work.
It could very well be your libpng15-15.dll file also.

But didn't you say it was working fine then started out of the blue to give you problems?
That could mean a filter conflict. If you added any other filters after the apng binary....Did you?
Do you have any other problems with other filters suddenly not working?

Or another way to get APNG through Gimp in Windows you can find here.
viewtopic.php?f=12&t=4467

I didn't explained well, sorry. I downloaded again and then I saw three files too.

I did download another tool called anitools. But I deleted it right away. I can't tell if that affected anything.

It only would if you downloaded another dll with it and overwrote an existing one for the apng plugin.
The program i linked above creates apng images from any GIF file. It works as a stand alone if you prefer to use it that way.If you want to call it through Gimp i provided a python file for that.Everything is explained in the readme_please file.
NOTE: This GIF2APNG setup doesn't allow you to open or save animated png files created in Gimp.
You can however create a GIF file in Gimp. Then SAVE it and call this execute with Tools>GIF2APNG and then create the APNG that way. However unless you have APNG plugin installed and working in Gimp you won't be able to open it in Gimp as layers. But XnView does open , and run APNG files well.

Hello there I'm currently experiencing some problems regarding APNG plugin usage in Gimp 2.8 under Windows 7 64bits, the problem is that I can't see the dialog, it is completely empty, only the export, help and cancel buttons show off, I've been digging around and the problem is within file-apng.ui, can someone help me out?

Here is what I actually experience

You need to make sure those two files ( the ui and exe ) are in the lib\gimp\2.0\plug-ins directory. They won't work in the user\.gimp-2.8\plug-ins folder.