In the following methods is most likely to used in this 
case:
Create your Req.Structure
Create the test case structure and the test cases
Map the test cases to the App.Req
Run and report bugs from your test cases in the test lab 
module.

The database structure in Quality Centre is mapping test 
cases to defects, only if you have created the bug from 
Application. test case may be we can update the mapping by 
using some code in the bug script module(from the customize 
project function), as per as i know, it is not possible to 
map defects directly to an requirements.

Test Cases are generally mapped with defects by going to 
the Defect mapping icon at the top of the running test set, 
generaly a red baloon like structure, also, u can create a 
new defect when the test case is faiing or you can add one 
exisiting defect by specifyig its id.

A defect can also be mapped in test lab failed test case 
from Defects module, mapping the same as specifying above.

From tes lab-- go to Linking/ Coverage-- go to Defects.. 
map the id or create a new defect.