Poke Transporter can only transfer Pokemon from the Generation V DS games (Black, White, Black 2 & White 2).
In order to perform your desired transfer, you need to first transfer your Pokemon from Platinum to Black/White/Black 2/White 2. This involves two DS consoles and your Gen V game should be able to access the Poke Transfer Lab.
From there, Poke Transporter should recognise your Gen V game and you can perform a transfer from Gen V to Gen VII.
Hope I helped. :)