This article will go into detail on how to install certificates on 2x Application Server
From the SSL/TLS tab of the 2X Secure Client Gateway Properties window, click the "..." link to browse to the Private Key you created during the CSR creation process, and then again to find the Certificate file that was returned to you. If you receive a certificate file that includes an intermediate (all certificates are issued with one or more intermediates for security purposes), you will want to combine those two files into one .pem file before enabling your certificate.
To create that file, simply open both certificate files in a text editor and copy them into a new file in the following format:
(Contents of your_domain.crt file)
(Contents of Intermediate Certificate File
You should be able to enable the certificate by browsing to your new certificate.pem file and selecting it like you selected the private key, and then pressing the OK button at the bottom of the window.