Android supports L2TP/IPsec since version 4.2x. If you have an older version, you need to choose another protocol.
Open the system settings.
 
Be sure you are in "Connections" tab and tap "More networks" menu item.
 
Tap "VPN" in order to get to VPN connections settings.
 
On "VPN" screen tap "+" sign at top right side of screen.
 
Fill VPN Connection form with next values.
    Name: Trust.Zone-Hong-Kong
    Type: L2TP/IPSec PSK
    Server address: hk.trust.zone
    IPsec pre-shared key: trustzone
         Trust.Zone-Hong-Kong
        hk.trust.zone
        trustzone
        Trust.Zone-Hong-Kong
        hk.trust.zone
        trustzone
    
Tap "Show advanced settings" and be sure that "Forwarding routes" is set to 0.0.0.0/0. Then tap "Save".
         hk.trust.zone
        hk.trust.zone
    
If all goes well, you should see new VPN connection, so tap it if you want to connect.
         Trust.Zone-Hong-Kong
        Trust.Zone-Hong-Kong
    
Put your username and password and check "Save account information" on if you want Android to remember your login credentials. Then tap "Connect".
    Your Username: HIDDEN. Log in or Sign up to see.
    Your Password: HIDDEN. Log in or Sign up to see.
         
        
        
    
If connection has been successfully established, you will see "Connected" near your VPN connection name as well as "key" sign in notification area.
         Trust.Zone-Hong-Kong
        Trust.Zone-Hong-Kong
    
Open Trust.Zone Main page or reload it if it is already open in order to check if your connection with Trust.Zone VPN is established correctly. 
You can also visit our Connection Check Page.
     Your IP: x.x.x.x ·
    
                                     Your IP: x.x.x.x ·  Hong Kong · 
        
            You are in TRUST.ZONE now! Your real location is hidden!
        Hong Kong · 
        
            You are in TRUST.ZONE now! Your real location is hidden!