Can Pulsed DC be used in a radio transmitter?
And a "AC" signal is needed to drive an antenna for electromagnetic
  radiation to occur, Is this correct?


If you are building a radio transmitter as per the title of your question then there are a few things you need to get right: -


You need to ensure you only transmit at allowable powers at allowable frequencies
You need to restrict the bandwidth of your transmitted carrier so that harmonics don't upset other radio systems at harmonics of your basic carrier wave.
You need to respect that an antenna presents an impedance that is quite variable to wideband signals and this can screw up the ability to produce wide band RF (see also 1 and 2)
You need to have an antenna that is designed for your output stage - you need to observe whether your antenna design is balanced (dipole) or unbalanced (monopole)


The AC content from the 555 is what an antenna is interested in - the fact that there might be a standing DC voltage on it that represents that average value of your pulsed waveform is of no-importance to the antenna.


  In the instructables project the "Pulsed DC" is directly used to drive
  the antenna. How does this work?


Quite badly and quite possibly illegally.