Setting Colour Point with curves CS4

[russellsnr]

Hi, I use CS4 and I want to do some colour correction so I use the colour sampler to select areas I want to change ie Highlights to around 242 but I want to place a sample point for midtones on the RGB channels at the same time, to do this I understand if I place the cursor over a sample point and hold Ctrl + Shift then click it should set points on all channels but when I hover over the sample my colour sample point picker changes to the move tool so I assume I must have hit a button someplace but don't no where! Any help please.
Thanks
Russ