I'm using v5.3.5 and as seen below there is no slider option under configurable options.
There should be slider option, it is also mentioned in help docs; http://docs.whmcs.com/Addons_and_Configurable_Options .
ss.jpg
In http://docs.whmcs.com/Addons_and_Configurable_Options
There should be slider option, it is also mentioned in help docs; http://docs.whmcs.com/Addons_and_Configurable_Options .
ss.jpg
In http://docs.whmcs.com/Addons_and_Configurable_Options