The vast majority of campaigns for Inkpact should be written on the side of the card that has the blank space for writing with the logo/footer. For anything different to this, it will be outlined in the job brief.
For greetings cards you should write on the inner right hand page like this 
Always check before you start writing that your card is the right way up so that you are writing on the correct side and the cards don't end up upside down!