use it any Pokemon that u have trouble catching u don't need it to catch mesprit or cresslina i used pokeball
After defeating Cyrus in Veilstone, he should give a Master Ball to you.
master ball
no
you can not get a master ball in Pokemon platinum only the one professor Rowan gives you
you cant get a second master ball in any game. The only one is given to you by Cyrus. If you have leafgreen etc. you can give one to a Pokemon to hold then transfer it to platinum by pal park.
with a master ball or timer ball
there is no more masterballs in Pokemon platinum but not if you use cheats
Master Ball.
with a master ball
You should use it on cresselia or mesprit because they run away
Master ball
You should use it on one of the 'roaming' pokemon, like Cresselia in Diamond/Pearl/Platinum.