I'm a newbie to this HTML5 Panel development and I'm starting to develop an extension for Photoshop CC 2015.
After I configuring everything right and run sample extension from Eclipse, it still returns "extension could not be loaded because it was not properly signed."
when I try to open it. This happens to every other development testing extensions. What should I do in order to not bump into this error?
Also I changed the My manifest.xml and debug mode 1 etc... but the same result happen.
I believe you need to install Chrome for debugging. Are you getting this when you are testing an extension or after you're done and trying to install it? You do need to sign the extension when you're done, or you will get this error. See this link about signing extensions:
Davide also has a book out on creating extensions.