How do you Switch to Multi -User mode?
i suggest you following steps to resolve this issue and you can easily switch to multi user mode :
.click file menue option and Open or Restore a Company File
choose company file and click Next.
you highlight the file name.
go to the bottom of this window, put check mark in the box Open in Multi User Mode.
Now each time you open this Company File from this computer, it will be in Multi User Mode.