As I said Media placeholder is for images, NOT shapes! And you have to have the image or the mask for the image (if you use that) selected. Shapes in Pages are not images.
In the Help menu You can click on *Pages User Guide* to download the guide. It will tell you this on page 259, among other things
+To create a media placeholder:+
+1 Add an *image, movie, or audio file* to the document. See “Working with Images” on page 133, “Adding a Sound File” on page 148, and+
+“Adding a Movie File” on page 149 for instructions.+
+2 Select the imported object and move it to the location where you want it to be in the document, as “Moving and Positioning Objects” on page 153 describes.+
+3 Format the imported object, as “About Manipulating, Arranging, and Changing the Look of Objects” on page 151 describes.+
+4 Select the object, and then choose Format > Advanced > “Define as Media Placeholder.”+
There is also the Help in the Help menu.
