Front derailleurs come in two formats. Band-on and Braze-on. Band-on have a band which attaches to the frame. Braze-on do not, they attach to a braze-on on the frame or, as FeltF2Tarmac says, to a band adaptor.
You need to determine if your original derailleur is a band-on or a braze-on with band adaptor. If it is the later then you should be able to attach a new braze-on derailleur to it. If it is the former then you will almost certainly require a band adaptor (or you should have ordered a band-on derailleur in the correct size for your downtube diameter).
As usual, a photograph of the situation would assist greatly.