I would like to modify the indicator to better fit my need, any way to do it?

If it's a Fidelity indicator, no source code is available. For a TASC Indicator, they're all open source now.

Although it's not possible to modify a closed-source indicator, you can always roll your own. Code it, debug, compile in Visual Studio and drop the assembly into Wealth-Lab main folder. An indicator, if it was properly coded to implement the IndicatorHelper interface, would then appear on the Indicator list. Here's more in depth details for Creating Indicator Libraries for Wealth-Lab.NET.

Downloading some source like Community.Indicators should also make it easier to start.

the above reference seems to be missing: http://wl4.wealth-lab.com/cgi-bin/WealthLab.DLL/getpage?page=WL5Indicators.htm

It's missing for a reason. There were dozens of discussions like this one and it's even on the WL5 Wiki FAQ.

Please contact Fidelity on this question.