The example of how to access leptons is given in examples/Example1.C (for electrons) inside Delphes3 installation directory.
The example of how to access missing ET is given in examples/Example2.C of the Delphes installation directory.
This is more complicated due to the presence of pileup and fake jets. In an idealized scenario, we will need identify “signal” jets reconstructed from non-pileup particles and apply an offset correction to compensate for extra energy from pileup. Here is simple method:
See the correction procedure here