im not really sure but you could try using the poke radar or migrat it from ruby or emerald hope my answer helps you.
where to catch lotad in Pokemon platinum i really need one to complete the national dex
I don't really think you can. The thing is, in Sapphire you can catch Lotad but not Seedot and in Ruby you can catch Seedot but not Lotad. However in Emerald, you can catch both. So You'd have to trade from a Sapphire or Emerald. Lotad can be found on route 114 on Pokemon RUBY!
If you mean a Lotad, then you can catch it in soul silver with specific items in it. Of course, it is a lot easier to transfer a Lotad via link trade or Pal Park from Diamond, Pearl and/or Platinum or Fire Red, Leaf Green, Sapphire, Ruby and/or Emerald respectively.
Catch it lol
After obtaining the national Pokedex insert Pokemon Emerald/Ruby/Sapphire into your DS then you can find it on route 203 & 204
u cant
The early routes.
Go to White Forest and Lotad will pop up some time Good Luck!:)
Pal Park one.
Yes you can find Lotad in Diamond but only with the assistance of putting your Pokemon Sapphire into the Gameboy slot of the original DS or the DS Lite at which point you wild come across Lotad in Routes 203 and 204, the north section of 205 and the south section of Route 212.
I don't think you can get one in the sinnoh region.
You only can find Lotad in Platinum if you insert the GameBoy Advance game Pokémon Sapphire in your Nintendo DS. If you've done this, you can find Lotad at Route 203, Route 204, Route 210 South and Eterna Forest.