If you have not yet created a Certificate Signing Request (CSR) and ordered your certificate, see
SSL Certificate CSR Creation | IBM HTTP Server.
IBM HTTP Server SSL Certificate Installation Tutorial
Before you can install your DigiCert SSL Certificate, you must install the Root and chain (or intermediate) certificates. After you have downloaded all three certificates, do the following.
Installing the Intermediate and Root Certificates
In UNIX, enter IKEYMAN from the command prompt. In Windows, start the Key Management utility in the IBM HTTP Server folder.
In the main User Interface, choose Key Database File. Choose Open.
Select your key database and click on OK.
Next, enter your password and click OK.
Click on the link to Signer Certificates in the Key Database content frame. Hit the Add button.
Installing your Primary Server SSL Certificate (your_domain_name.crt)
In IKEYMAN, in your Key Database, click on Personal Certificates, then the Receive button.
Choose your DigiCert SSL Certificate (your_domain_name.crt) from the Receive Certificate from File Box. Then click on OK.
For further assistance, check with IBM, as they have a helpful guide for SSL Installation.