Before installing any software in the E61i go to
Menu> App.Manager> Options> Settings> Software Installation

In ”Software Installation” make it ”ALL”. This change is important, otherwise you will get an Error message while installing most freeware on the Nokia E61i.

Now, here is the way I install themes and other applications in my Nokia E61i. I use the USB Cable to do the installations.
1. First install the Nokia PC Suite software on your computer from the CD which you recieved with your Nokia E61i.
2. After Nokia PC Suite is installed in your PC, connect the Nokia E61i to the PC with the USB cable which was provided in the box.
3. When the USB cable is connected, Nokia PC Suite will recognise the E61i. Then on the E61i screen you will see an option asking if you want to connect via “PC Suite” or Data Transfer. Choose PC Suite.
4. Now, simply download the .sis or .java files (themes & applications) to your computer and “Right Click” on it with your mouse. You will see an option saying “Install With Nokia Application Installer“.

5. Click Yes on the PC, and then Yes & Continue on the Phone when you see the prompts.
6. The phone will then ask you if you want to Install the Theme or Application in the Phone Memory or Memory Card.
I strongly recommend Installing themes in the Phone Memory! This helps in avoiding complications later on when Uninstalling.

When I first bought the Nokia E61i, I made a mistake which a lot of new S60 3rd Edition users make, that is, I installed the Themes in the Memory Card instead of the Phone Memory. This was a huge mistake!
There is a bug in the Operating System of all S60 3rd Edition phones, due to which, if the theme is installed in the Memory Card, it cannot be deleted or removed, as it then does not show up in the Application Manager.
Ever since that mistake, I have made sure to always install themes and softwares in the Phone Memory, instead of the memory card. Yes, it eats up space on the phone, but it’s worth it, as then it is easily removed as well.
And that’s it, the theme or application is now installed!
If you want learn how to uninstall a theme or software, click on the following link
http://mobilepakistan.wordpress.com/2008/01/11/removing-themes-softwares-in-e61i/