more questions about signal tap

I would assume you already have a clock. Signal tap simply samples the signals you tell it to sample into on-chip memory using the rising edge of the clock you tell it to use. If you have a signal tap working, you would have had to give it a clock. The non-gated guidance tells you a little more about how to get signal tap to work right. Any signal can be made a "clock" simply by using it to drive the clock input of a register. What is normally thought of as a clock is one of these clock signals that is also driven by a clock buffer, so you can think of it as having almost negligable skew across the part. Then it can be thought of as a typical clock domain. More specific than that, signal tap wants you to use either an input pin (assumed to be driven by a clean clock source), or an output from an on-chip PLL. A register output would also qualify as non-gated, but then you would need to ensure it gets on a global net. Signal tap does not want you to derive a signal with combinatorial logic and call that a clock. That would messes up the timing.