Hi,
Is there a specific reason you want Pro Tools to slave to Cubase other than just hosting the video? If there isn’t, I highly recommend not using this setup. Cubase and Pro Tools don’t interact well concerning syncing just for video playback. Pro Tools will slave to Cubase via MTC, but it doesn’t work with MMC which is what you’d need for incremental adjustments (ie, stepping one frame at a time). You’d be much better off just keeping the video in Cubase. Or, if you can afford it, there is an application called Video Slave by Non Lethal Applications which runs perfectly with Cubase.
If you DO need pro tools to slave to Cubase, let me know and I can walk you through the processes.