If you also want to access aMPdroid over your data connection (not only wifi) you can either forward all the necessary ports (defaults: 4322 for MPExtended, 8017 for WifiRemote) or use a VPN connection to directly connect to your local network.

VPN access has the advantage that it is secure and relatively easy to setup when your htpc runs under Windows 7.

Setup Window 7 PPTP VPN

One problem with Android and VPN is that the android VPN client is currently not very mature and has problems connecting to many VPN servers (e.g. Windows Server 2008 VPN Server). Fortunately Windows 7 also has the (little known) feature to run a PPTP server (and even better one that Android can connect to).

A detailed tutorial can be found in this article, here a short summary:

1. Go to the Network Connections section (Start -> Control Panel -> Network and Sharing Center -> Change Adapter Settings).
2. Press alt to show the menu bar. Select File -> New Incoming Connection.
3. Select users that will have access to the VPN connection.
4. Check the box Through the internet.
5. Click Allow Access.

That's it. Your VPN connection is setup and running. You will need to forward the PPTP ports (1723 and 43) in your router settings so it can be accessed over the internet.

Setup a dynamic dns entry

Since most people have dynamic ip addresses, you will need to setup a so called dynamic dns address. There are free services out there (e.g. http://www.no-ip.com/) that will help you setup a dynamic dns name (e.g. myhtpc.no-ip.com) for your htpc.

Use your favorite search engine for detailled information on how to setup dynamic dns services.

Setup and connect to your VPN server with Android

1. Go into the Android settings -> Wireless & Network
2. Open VPN Settings
3. Select Add VPN
4. Select Add PPTP VPN
5. Fill out the VPN settings:
   1. VPN name: Name of this VPN Connection (e.g. MyHTPC-VPN)
   2. Set VPN server: The ip address or dynamic dns name of your htpc (e.g. myhtpc.no-ip.com from above)
   3. Leave the other two settings
6. Press Menu -> Save

You should have a new VPN with the name you entered before in the VPNs list. Click on the VPN and enter the username and password of one of the Windows 7 accounts you gave access to your Windows 7 VPN server. If the connection is successful, your mobile is directly connected to your local network and will behave excactly the same as if you were connected through your wifi router at home. You should be able to use the same aMPdroid client you use at home now. Be aware that your mobile data connection will be much slower than your local wifi network.

Faster Access to your VPN on Android

Android unfortunately doesn't make it very user-friendly to connect to your VPN connections. There is no way to save the password (has to be entered every time) and to connect you have to go into the settings.

A very good alternative are the apps from developer DOANDROIDS: 5 VPN (free) and 1 VPN (paid). These apps let you store your VPN credentials and connect to your VPN in 5 or 1 clicks.

The more advanced (but paid) 1 VPN will give you a nice looking widget with which you can very easily toogle your VPN state.