Installing the official Helix Resource Pack.
The Helix Resource pack is required in order for some features on the server to work properly. The texture pack will automatically prompt you to install it when you join the server for the first time. When joining the server other then your first time, it will be required in order to play.
To enable the server resource pack, edit the server under the Minecraft Server List, and make sure "Server Resource Packs" is Enabled.