I have created my own node that I have exported. I would like to be able to import on a Knime version I use for data analysis using the Help->Software Updates->Find and Install… If I choose New Local Site or new Archive Site it says: Does not contain update site. Am I missing an xml file to set up my own site?
For being able to install new nodes via the Update Manager mechanism, you
a) need a feature the contains at least the plugin where your node resides (features are containers for a bunch of plugins distributed together)
b) a site.xml that contains a short description of the feature and a link to the JAR file containing the feature.
So, if you have not already created a feature, you need to add a new feature project and add the plugin containing your node to the feature. Then you need a site.xml (there is a wizard in Eclipse for creating them, you may also have a look at KNIME's site.xml at http://www.knime.org/update/site.xml). Finally you need to export the feature (not the plugin!) as a "Deployable feature" into an archive file and put the site.xml into the root of the created zip file (same level as the plugins and features folders).
If you just want to test the plugin you may also just copy the plugin JAR (or the folder, depending on how you exported it) into the plugins-folder of the KNIME installation. However, then you will not be able to automatically update it.