If you can't install AdBlock from the Chrome Web Store, first make sure that Chrome is updated to the latest version. Type about:chrome in the address bar. Chrome will check for updates and tell you if one is available.
Next, try restarting Chrome, then try to install AdBlock. If it doesn't work, try two or three more times. Sometimes it takes three or four attempts to flush whatever is preventing AdBlock from being installed.
Next, try restarting your computer, then try to install AdBlock. A reboot solves 95% of all known computer problems.
If Chrome is fully up-to-date, you've tried restarting both Chrome and your computer, and you're still getting an error when you try to install AdBlock from the Chrome Web Store, which error are you getting?