Probably you still have the category definition within your plugin.xml file commented out. You should use as a template to create new. (Be sure you include in the binary build its icon!)
As for the category: I think the non-KNIME nodes cannot add to existing (closed) categories, you can create your own or you can add under /community.
<!-- This is an example for providing an own category -->
<extension
point="org.knime.workbench.repository.categories">
<category
description="Category containing my nodes"
icon="icons/default.png"
level-id="myNodes"
name="My nodes"
path="/"/>
</extension>
You have to set the category-path of your node to "/myNodes" (if there are multiple nodes in that category, it is also ok to add after property with the id of an other node).
You can put anywhere your icon in your project, just be sure it gets exported to the binary build (see the Build tab when you open META-INF/MANIFEST.MF or the plugin.xml file with the Plug-in manifest editor) and is properly referenced in your plugin.xml file.