Sure ill take a crack at it. First off open up wiimc and before doing anything else goto settings > online media and look at the value for online media folder. It it is anything other then blank just click it and clear all the text. click go back > twice and you should see a screen that says saving. Once that is done try clicking the online media icon again. If you still get the same error message i would next suggest taking a look at apps/wiimc/onlinemedia.xml in your install folder. Mine contains the following excluding vlc shares.

Make sure your file looks simmilar and if everything looks good and your still having trouble then hopefully someone else will have an idea.