I have just purchased the upgrade from Poser 12 to Poser 13 and I always render my animation with png files then import them into Final Cut Pro so as to give me a clear background, it seem that the option for rendering to png files is now missing any ideas what I can do please

Poser now defaults to making movies from images and keeping those images.  In the past it would make the images then make the movie file and then delete the images.  You can still do that using the bottom 3 check boxes.  However, in your case you have to first choose a file location which is the image path right under the Render type.  Click the change button and then set your path where you want to save the images to.  As is normal you can also choose the type of file right below the pathway dialog box.  Standard option's PNG, TGA, Tiff, BMP, JPG, PSD.  So, you set the type of images to save to when you set the image storage pathway.

Part of the improvement in 13 is that you can now render to images (always did that but would Auto delete unless to frames) but under the video Export options choose to encode the result in several different formats without having to re-render the scene multiple times.  So, to get the same basic functionality you were used to click the 3 check boxes. Auto make, Auto delete frames and Autoplay movie. I would not recommend the Auto Delete frames as you would then have to re-render the scene again.

The other nice thing depending how you view it is that the frames are now stored along with the PZ3 file.  And you can setup a different folder for each scene, to keep things grouped together.

just realized you must be on an Apple. On windows it is a simple matter of changing file types in the file/folder selector dialog box.  On Mac not sure if they give you that option.  If they don't you may have to file a ticket.

I see what you mean. I made use of changing the export movie format to Images Files myself in Poser versions prior to 13.

I'll give what Richard60 has described a try.
On Mac it does still have the Image Path option, and if you click Change you can change File Type. So sounds like it's the same regardless of OS.