Can I access my Time Capsule remotely if I haven't set it up at home?
If You Have a Time Capsule or an AirPort Extreme Base Station with Shared USB Hard Drive 

If you have either a Time Capsule (which is basically an AirPort Extreme Base Station with a built-in 500 GB or 1 TB hard drive) or an AirPort Extreme Base Station (AEBS) with an attached USB hard drive, you can share out the Time Capsule/AEBS hard drive and make it accessible via the Internet. To do this: 


Start the Airport Utility. 
Select your Time Capsule or AEBS. Make a note of the IP Address shown on the right -- you will need it later. 
Click Manual Setup. 
Check your "Connection Sharing" setting under the Internet Tab. The following tutorial is valid if your "Connection Sharing" is to "Share a public IP address", the normal setup for a home network. You will need to have a static IP address, or use a free dynamic DNS service. If you have a different type of "Connection Sharing," you probably don't need a tutorial to set up remote access to your disk; adapt this one as needed.
Click Disks (at the top of the dialog box), and then click File Sharing. 
Select (check) the "Enable file sharing" checkbox and the "Share disks over Ethernet WAN port" checkbox. It is strongly recommended that you also set Secure Shared Disks to "With base station password" and Guest Access to "Not allowed"; not making these changes may allow unauthorized users to access your Time Capsule/AEBS hard drive. 
Click Airport (at the top of the dialog box), and then click Base Station. 
Enter a Base Station Password and verify it in the Verify Password box. 
Click Advanced (at the top of the dialog box), and then click Port Mapping. 
Click the plus sign (+) to add a new port mapping. 
In the Public UDP Port(s) and Public TCP Port(s) boxes, type in a 4-digit port number (e.g., 5678) that you choose. In the Private IP Address box, type the internal IP address of your Time Capsule or AEBS that you wrote down in step 2 (for example, 192.168.0.1). In the Private UDP Port(s) and Private TCP Port(s) boxes, type 548. Click Continue. 
In the Description box, type a descriptive name like "Time Capsule File Sharing" or "AEBS File Sharing". Then, click Done. 
When you have made all changes, click Update. 


Your Time Capsule/AEBS will restart. Once it does, you are now ready to connect to the Time Capsule/AEBS hard drive via the Internet. To do this when your MBA is away from home: 


In the Finder, click Go > Connect to Server. 
Type in the correct domain name or external IP address for your network, plus a colon and the port number you specified in step 11. For example, "www.myhomedomain.com:5678" or "123.123.12.123:5678". 
Click Connect. 
You will be prompted for your user name and password. The user name can be anything you like; the password should be the password for the Time Capsule/AEBS which you specified above. 
Click Connect. 


Voilà! You are now connected to your Time Capsule/AEBS hard drive from your MBA. You can access files, copy files back and forth between your MBA and the hard drive, delete files, whatever you want, as long as your MBA remains network-connected. The next time you go to connect, it should go even more quickly (especially if you save your password in your keychain, and if you add your home IP address/domain name to your list of Favorite Servers in the Connect dialog box). 

Note that the Time Capsule/AEBS will appear in the Shared section of your Finder's sidebar as a server, with the Base Station Name of the Time Capsule/AEBS as the server name. 

Taken from http://forums.dealmac.com/read.php?4,2800627