To get into Safe Boot it depends on which version of DellEFI you are using:
DellEFI 1.1: just after you turn on the machine after the dell logo, you should see it count down: (3), (2), (1).. while it's counting, press any key. This will bring you to a command prompt your where you can type: -x (then press enter).. you should then boot into Safe Boot..
DellEFI 1.2a5: just hold shift while it is booting (it is easiest just to hold shift then hit the power button and continue to hold shift until you see the Safe Boot login screen).
Please keep in mind that Safe Boot can take a bit longer to load/boot as compared to a normal boot.