To use this PRO feature you will need a ".WAV" file. It should be an uncompressed signed 16 bit PCM mono 8000 Hz one, without any headers.
When using Windows sound recorder to prepare such a file it will be created as 32-bit float for example. This will not work with Zoiper.
You can use most audio editing or conversion applications on the market. We can suggest the free, open source and cross-platform Audacity.
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.