For some reason I am unable to build or destroy blocks. I've seen videos and I know what it looks like... but I don't see that. I don't see a block in front of me and blocks I am pointing at don't light up. Right click/left click does nothing. Scroll wheel changes the block type, but that doesn't matter much if I can't build.
There is a way to check if it's a hardware issue (with your mouse).
I'm assuming you use Windows other wise you'd have specified.
Tutorial To Setting Up MouseKeys
If you can Dig/Build using this, then there is some form of hardware issue with your Mouse and JavaApplets.
This problem might be resolved with the downloadable client. (When it's released)
Nadda.
OS and Browser specs.
Firefox 3.0.11
Windows XP