The name has changed; you can now find the new button at the bottom left on the screen just below the last post. Here is a screenie showing it. Also, just for the record, only the "thread creator" or the moderator/admin team can mark threads as solved. It seems that you posted the question using your facebook login and now want to mark it as solved by using your old Daniweb ID. I'm not sure if that's possible but Dani should be able to clarify on this given that facebook login was recently added.
If you have two separate logins, one old one, and one through FB login, then they are entirely unrelated and there is no way to merge them.
If you wish to be able to use the FB login feature with your original account, then change the email in your member profile of your original account to the email you have on file with Facebook. When we try to authenticate you when you login with Facebook, the way we know what account to log into is based on matching emails. If there were no matching emails, a new account was created, which is why you ended up with two accounts. (Note that you will also need to change the email associated with your new FB account to something different since no two accounts can have the same email address.)