Hi Stendor,
I'm sorry to hear about the problems you're having with the installation of metaPost. I'll certainly do anything I can to help. I haven't seen this particular error before, but I"m sure we'll get it figured out.
Regarding the visibility of the providers, they are not visible until you've requested a module during the account setup process. Another way to say this is that the providers show up in metaPost after they are first requested through the account creation process in Windows Live Writer. We've written the metaPost infrastructure so that the modules are detected at runtime. The advantage of this design is that you can add third party modules which support the metaPost publishing interfaces and metaPost will pick them up automatically. No configuration is necessary.
All that to say, the providers will show up as they are used.
Based on the example you gave above, I would have guessed that at least the text/html provider would show up in the list. The only other thing I would check is to make sure that the portalalias for the portal you're connecting to matches the portal you used in the posting URL above. If you include www in your portal alias, you'll need to include it in the posting URL.
One last thing I notice about the posting URL that you've posted is that it's missing a sid parameter to allow you to pick up the styles. See the documentation (In the resources zip file and under a folder called documentation) for more information about configuring the text/html module.
Also, please don't hesitate to call or send me an email. My contact information is on the Contact Us page and my email is don at itcrossing.com.
Thanks for using metaPost!
Don