When a new version of a Safari extension is released it has to be approved by Apple before it's allowed in the Safari Extensions Gallery. Apple's review process is lengthy and somewhat arcane. While an AdBlock update is in review and not yet available in the Extensions Gallery, you can download it from our website at getadblock.com.
However, Safari may show you a warning like this when you try to install AdBlock this way: "Are you sure you want to install the extension “AdBlock”? This extension is not from Safari Extensions Gallery. You should only install extensions from sources you trust."
If you see this message, first try clicking Trust. If that doesn't allow you to install AdBlock, restart your Mac in safe mode, run Safari, and then you should be able to install AdBlock.