I installed two browser plugins successfully (Adblock and Useragentswitcher) but cannot update adblock list, resp. set user agent. Both components are listed in the components list.

In the latter case on the site http://browser-extras.garage.maemo.org/news/8/
there is a link

"Get the installer here and check it by yourself. Remember to restart your MicroB browser twice and then configure it according to your preferences by clicking here or manually loading chrome://useragentswitcher/content/config.html. "

I did this (click on 'here' or 'chrome...') but nothing happened.

What am I doing wrong ?

Well, when I open any page over http in the MicroB I get a tiny red square at the lower-right corner of the page. When I click it I can configure AdBlock. I hope it helps.