On my slider, to boot into CWM I do the following:
1) Turn tablet completely off, and wait 15-20 seconds to ensure its 100% off.
2) Press volume down, and without letting go, press power button.
3) Keep pressed until tablet turns on, then release power (but keep vol down pressed)
4) You will see some white text on the top left corner of the screen. When you see this, let go of vol down and press vol up.
If instead of CWM you see an android with an exclamation point sign, then CWM is not installed or not installed properly on your system.
Hope this helps...