You have to make sure that QuickBooks Desktop is downloaded on the server's computer. In step 1, you will only select I will NOT be using QuickBooks Desktop on this computer if you just want your server computer to host your files. This option does not allow you to work on QuickBooks from the server computer. Then, follow the on-screen process to install the QBDT.
This way, you'll be able to turn on the Multi-user Access on your server computer in step 7.