That snippet is really old. flaskext is no more (or at least very deprecated). Refer to packages directly rather than through flaskext or flask.ext.
from flask_sqlalchemy import SQLAlchemy
Flask-SQLAlchemy (and most other extensions) no longer register in the flaskext namespace, and flask.ext was deprecated then removed in 1.0. The only correct way to refer to extensions now is by directly importing them.
If you’re still getting an import error, then you didn’t install the package in the same Python environment you’re running. Make sure you’re using a virtualenv.