To use this PRO feature you will need a ".WAV" file. It should be an uncompressed signed 8, 16, 24 & 32bit PCM mono, without any headers.
You can use most audio editing or conversion applications on the market. We can suggest the free, open source and cross-platform Audacity.
You can also find Audacity's manual on their website.
To set and enable the custom ringtone you need to go in Zoiper -> Settings -> Accounts -> Select your account -> click on "Advanced" and scroll down until your find:
Note, this option is "per-account", which means that you can set one custom ringtone for any SIP or IAX account that you have set with Zoiper. Custom ringtones per-contact are currently not supported.