Did you try option 3? The password recovery option should help you reset the password. The user guide says that the password recovery option will allow one time access to the device from local terminal with no password entered. You will then be able to manage the users.
Let me start with, I have absolutely no idea what I'm doing! I received a previously used\configured PowerConnect 3548p that I'll be setting up in a home test lab.  I have my console cable attached, launched PuTTY, fired it up, and get output.  It wants a username\password that I just don't have. I interrupted normal boot to get into the Startup Menu.  Erased Flash File, then entered a name.  Below is the output on the startup menu I received. I hoped this would set the password back to a default.  I have tried admin:password, admin: , root:calvin.    [1] Download Software[2] Erase Flash File[3] Password Recovery Procedure[4] Enter Diagnostic Mode[5] Set Terminal Baud-Rate:emotion-14: Stack menu[7] Back Enter your choice or press 'ESC' to exit:Warning! About to erase a Flash file. Are you sure (Y/N) ? yWrite Flash file name (Up to 8 characters, Enter for none.):defaultFile default (if present) will be erased after system initialization. Exit from boot menu. Continue with flow.
Dell PowerConnect 3548p factory reset to reset login credentials