You have four tabs in QC 
Requirements
Test Plan 
Test Lab 
Defects 

When you go to Test plan you can see your Test Cases which you loaded/written in QC .
Just Select set of Test Cases u want to map in Click on it and in left top u can see Requirements tree and easily select.

I know this is not detailed steps , i will try to get u detailed step in day or two

Step1- Select the test cases from Test Plan tab in QC.

Step2- Select "Requirement Covg" tab, it will show option for "Add requirement( an umbrella symbol)"

Step3- Click on that "Add Requirements" option, it will open Requirements pane at right hand side.

Step4- Select the appropriate Requirements and click on ok.
  (Sometimes it will ask to add requirements with its child contents, then u may add it.)