Click on Manage on the VPS for which you wish to see the addon/additional IPs.
Switch to the Manage Network tab. (Please note, the user-interface design may be different on your side, but the tab-name will be the same, "Manage Network").
Here, You will find the IP Addresses allocated to your VPS, their Netmask and Gateway Addresses as well.
If you are on a OpenVZ Based VPS, you can automatically "add" (configure) all IPs to your VPS interface by clicking on the "Reconfigure Networking" option.
If you are on a KVM based VPS, You would need to manually configure the addon-IPs to your VPS interface as needed, we have guides available to help guide you through the process for this,
If you are using CentOS (6, 7 or 8), please refer Configure additional IPv4 to CentOS.
If you are using Debian or Ubuntu please refer Configure additional IPv4 to Ubuntu.
If you are using Ubuntu 18.04 and beyond please refer Configure additional IPv4 to Ubuntu using Netplan.