When using the Add to MasterCook bookmarklet to import a recipe, I typed in my user name and password, but it repeatedly asks me to sign in and won't accept it.
First, make sure you are using the correct user name (email address) and password. Try it from this web page:
Are you able to sign in at the web page above using the same email address and password as you are using in the web import window?
Sometimes it is a web page itself that causes the issue. Did you try other recipes on the same website? Did you try another website? Usually this happens because the web browser is no longer accepting third-party cookies. A cookie is set to remember you log on credentials. If a cookie can't be set, it behaves the way you explain. Something is blocking that from happening. You might double check your third-party cookie settings. Also, if you are browsing in privacy mode that doesn't allow cookies either. Furthermore, if you are on a website that requires its own login, that could conflict. I would suggest you empty the temp files for your browser and also check your cookie settings. See these articles:
See if it works on this web page too: