Before trying to do this, make sure you've sent a ticket to our support desk to try and find a solution for the reason you can't install FreeSO with the installer available at http://freeso.net.
However, if you need to install it manually for whatever reason, here are the steps:
1. Download the TSO installer provided by EA's servers. This will install all the original TSO files which are necessary for FreeSO to function. This installer will most likely not give you any problems at all while downloading. http://largedownloads.ea.com/pub/misc/tso/Setup The Sims Online.exe. Install it in the C:/ drive.
2. Follow the installation steps, when it asks you for a CD Key simply cancel the installation. You will already have the necessary TSO files installed.
3. Download the latest FreeSO version from our servers: http://update.freeso.net/dist.zip.
4. Extract the contents into a folder wherever you want (My Documents/FreeSO is recommended), and create a shortcut on your desktop for easy access.
5. Make sure to install OpenAL which is required for the game to work. https://www.openal.org/downloads/oalinst.zip
Done! You should be able to play FreeSO without any problems.
Your next question will most likely be: "How do I change the game window resolution?". Check out this tutorial here.