Send an email to support@bigbox-3d.com. You can proceed with your assembly without it and just add it on once you're finished.
Weird. I am also missing that part in my BigBox Dual package. I might just print another one on my Printrbot if you guys could provide the .stl. Are the printed parts files available yet?
I was tempted to put together the machine as a single (i.e. non-functional second extruder) then print out the part, but given that it arrived in 3 days via fedex, it was just easier to get the part sent.
My Dual kit is also missing the same part (I've just emailed support about it). Will soldier on without in the meantime
The good news is that there are tons of other tiny assemblies to keep you occupied in the meantime...
Oh, wait, I beat you. I just found my second missing part. THE ENTIRE BACK FRAME! The little idler, sure, it's the size of a stick of gum and is easy to leave out, but this is the size of a large poster on a wall... (already emailed...)
Oh dear, guess what I found as we went over the BOM for the acrylic parts this morning... Yup, a missing back frame! We'd done a visual inspection of all the acrylic pieces yesterday, because I'd spotted a crack in the heated bed bottom (which support were emailed about y'day), but hadn't noticed until this morning that we'd not ticked off the back frame on the BOM!
hmm sounds like with their rush to get the orders out they are slipping a little. Hope they correct this issue. Seems to be a recurring thing recently.