Hello,
1) Reseller Panel -> IP Assignment. Make sure that they're all "Free" and not "Shared" so that when they get assigned, they'll be owned and not shared.
2) When creating a User you should see the IP's in the dropdown. Just select the free IP you want. If you don't see the IP, then it hasn't been assigned to the Reseller.
*Note, IP's are bound to users, not Domains .. meaning a user can only have 1 IP address. If you want multiple domains with owned IP addresses, you need to create 1 user for each domain.
John