Try looking in your custombuild. I believe if this isn't unstalled you won't be able to let resellers have the WP option. I've installed it and I have it available in my reseller options
Alternatively, for around $2.50 a month, get a Softaculous licence. That has a decent WP manager built in and will give your resellers and customers much more control over WordPress installations. I have it on all my servers and it offers around 400 instant installs and is very popular in the hosting community.
If you have a legacy license, it's not present.
If you have a modern license, then change to Evo skin and see if you can find it there like in the screenshots of @DrWizzle is shown.