When I try to install it gives me this:
http://img7.imageshack.us/img7/356/capturehcu.png
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Hi! I had a simillar problem. The extension was installed but couldn’t be activated. Coincidence or not, it was solved after I uninstalled AdBlock Plus.
Responding to a very old topic is unwise as a lot can have changed in 8 months.
The avast AdBlocker was based on AdBlockPlus and if AdBlockPlus was installed then it wouldn’t install AdBlocker.
That said I don’t know what version of avast! you are using (latest avast! 2014 - 9.0.2008) and as far as I’m aware that no longer installs the AdBlocker add-on.