Can I suggest that you deflate the tyre, push the bead in on both sides, then add air until the tube has barely taken its proper shape. This way it won't jam between the tyre bead and the rim. Make sure the tube isn't pinched anywhere, and push the valve stem in most of its length (with the nut on, of course), lube the bead both sides (soapy water will do), and pump the thing up.

It should be ok. I have used MT21s before and don't remember them being troublesome, but things change...