Currently the new Edge extensions are considered as previews and these are only available in the Windows 10 Insiders. Also you might need the Build 14291 to use them.
Delivering extensions to Microsoft Edge and other features by Windows 10 Insider Preview Redstone Build 14291
This has been one of the long awaited extensions of the Microsoft Edge and even pinned tabs and other Edge enhancements to it.
Installation and usage of extensions in Microsoft Edge of Windows 10
Recently, the software giant did promise of giving extensive features and add-ons like Adblock Plus, AdBlock, LastPass and Amzaon, but it only provided three less exciting features.
Installing Microsoft Edge and its extensions on Windows 10
Even though the software giant had promised to provide a huge features and adds-ons like Adblock Plus, AdBlock, Amazon, Evernote and LastPass, but it launched the new product with less features.
Latest update of Windows 10 – Windows 10 Insider Build 14291 for achieving user-friendly environment
This extension support is what Microsoft wanted and it provides an edge for it in order to compete with the likes of Chrome and Firefox.