The key is always the same for Apache, Nginx and IIS. It's the one you got at the very first when you created the CSR:
Note: DO NOT DOWNLOAD THE KEY DIRECT FROM YOUR PRODUCT PAGE, IT WILL CAUSE ERROR IF YOU DO!!!
Apache requires a certificate file(1) and a CA chain file(2) which you can get on your product page as below:
Nginx PEM file is please check the tutorial here.
For Windows IIS you can create a PFX from the key/certificate file/Chain File use the following command in your SSH:
openssl pkcs12 -export -out your-domain.com.pfx -inkey your-domain.com.key -in your-domain.com.crt -certfile CA-chain.crt
Make sure to replace your-domain.com above with your actual file name and related contents.
For TOMCAT users please check the tutorial here.