I downloaded Buildcraft directly from its developer's site, and I placed the .jar in the mods folder I created in the .minecraft directory.
But first I downloaded the Forge APIs from this site, first the recommended (1180) iteration. I ran the Windows installer, which created a profile in the Minecraft launcher. Then did I add the Buildcraft mod. I ran the launcher, but only the three Forge "mods" showed. I then downloaded Forge 1186 and again ran the launcher with a new profile. Still no success.
This is exactly the same procedure I had previously used with many, many mods and the many iterations of Forge I had successfully run with Minecraft 1.6.4.