This can be done by tracibility matrix.

First go to the test lab and select the test case that 
needed to be mapped. On the right hand side of the test lab 
select the requirement coverage tab and click the select 
requirement tab, requirement tree is displayed on the right 
side. You need to search for the requirement that need to 
be attached or you can find by requirement name in the 
find  box. Then click on the add to coverage( This is shown 
in the requirement tree with the arrow). Thsi will add a 
requirement to the test case.Now you can close the 
requirement tree.

Whenever requirements are mapped our first aim should be to 
map all the requirements. We should make sure that atleast 
one test case is mapped to each requirement. Start from the 
requirements tab and through the click of relebvant buttons 
we can view the Test plan tab and locating the right test 
case from the Test plan and mapping it to the requirement 
will make the requirement mapped and we can also view 
whether all the requirements are mapped by viewing the 
status of the requirement through various views