It works a bit differently in RQM 2.x and RQM 3.x. In RQM 3.x you can link multiple testcases to a requirement at once.

When you create a requirement in RQM 2.x it will be stored locally in the RQM database and managed in the RQM web UI. To associate the requirement to a test case open that test case in the editor and click on the Requirements section. Then click the '+' button to add one or more requirements to the list.

When you create a requirement in RQM 3.x it will be stored in the RM database and managed in the RM web UI. To associate test cases to the requirement open it in the RM web UI and go to the Links section. Next click the Link icon (looks like a chain link) and select Validated By. Select the test case(s) that you wish to associate to the requirement. After performing these steps you will also be able to see the requirements in the Requirement Links section of the test case.