The title pretty much says it all
The online material library was originally written to work with older versions of blender, for more recent blender versions the addon has been broken since the python api relating to nodes and groups changed. As the author has had more important things to do it has been neglected for a while, but I understand he plans to get back to it soon.
I have submitted an update that is waiting to get approved and merged into the official contrib repository, until then you could use my patched version. While I did make a change to get group exporting working, there seems to have been another change that broke group exports so only materials without groups are working at present. I don't think any of the available library materials have groups so it would only affect materials you export.
You can download this one file that was changed and use it to replace the
addons_contrib/online_mat_lib/__init__.py that you have now. Another option is to download my entire contrib repo (about 20MB) which also has other addons included and copy the
online_mat_lib folder (or all of it) into the