Hi Aabid Baba
I believe you would be well aware of the theory and basics behind add module, drop module or add/drop multiplexer but since you want to know about the use of these modules in some example that would be existing in the examples. I have myself come across an example in which these modules were used . In examples of optisystem 14 go in the WDM system section and in there u would find an example of COADM and in that u will find that how add, drop modules are used. See for them if u find them well and good if not mention it i will upload the .osd file for you in that case.
Regards
Burhan