In v20 addons are stored inC:>Documents and settings>User name>My Documents>Blockland>Add-Ons.
What add ons are you downloading exactly?Some add ons will not execute because they were considered as crap-ons.If they are not crap-ons and if they still do not work, then they are definitely not in the right folder.
1. no description.txt in them2. you extracted them ruler this one out, he said he did it before successfully3. you put them in the wrong place