The ticker for Dell is ... DELL
It depends on the software that you are using, but some need you to put the exchange in as well. I presume you have looked at the Help for how to select stocks, or looked at the examples they use.
Each equity has a ticker set by the exchange so that everyone uses the same, BUT it depends on the data feed that your software uses as to how that ticker will be displayed.
For example, ES03M, ES3M and ES`M are three versions for the ES ticker (emini Dow futures).
BA. in the UK is for BAE Systems, but BA in the US is for Boeing Aircraft Corporation. BAY in the UK is British Airways, but BAY in the US is Bayer.
So you need to check your software's requirement for adding the exchange in as well for it to locate it.