If you know what you are doing, you can perform the following steps to install SMS on your PS2.
Download the latest zipped SMS.ELF binary from http://home.casema.nl/eugene_plotnikov/
Unzip the SMS.ELF file to a local folder on your computer.
Copy the file to your PS2's memory card (via ExecFTP or FileBrowser + USB, ....).
Restart your PS2 and run the SMS.ELF file from your memory card.
If the above doesn't make any sense, please read the next section.