You need to back the locking knob out a long way, until you can't see it up the mount hole, then the bracket should slide down on the stand the full way, it should cover all the silver top portion of the stand, then tighten the locking knob.
Thanks for the info , have just tried that but it still does not go the full way as the top of the stand screw hits the bottom of the brackets swivel part meaning the screw that secures it to the stand is sitting only half way contacted on the shaft edge it seems secure but looks odd maybe its just the way it is ! Once the umbrellas arrive if they ever do will have more of an idea of how solid it all is.
In that case you need one of these.
It is a female spigot with a 1/4 - 20 thread in one end and a 3/8 - 16 thread in the other.