Go to the Requirement module >> Open any requirement to which you want to link the test cases
Click on the Requirement toolbar "Link" >> "Test Cases" >> "From Grid" or "By ID"
In the "Link Test Case to Requirements" window, select the scope of the test case you want to see in the Grid
Select the Test Cases from the grid >> Click "Link Test Cases" >> Click "Close"
Now you can see the linked test cases in the Requirement >> Test Cases section