Trouble using mask tool and filter...Could not rasterize because the pixel data for this layer is not directly editable - masking

Question

I've searched for an answer to the problem I'm having on cs4 and can't find it, so here goes:
New to photoshop,  I'm trying to fake some depth of field (i.e blur the background) on a wedding photo by using the masking tool to mask out the foreground then, out of mask mode> select>inverse to get the dotted line around the image, then using Filter>gausian blur on the background.
1. The dotted line around the image doest look right
2.  I can't use the filter facility (its greyed out) and when I try to use the blur tool from the side menu got the above message.
Any help much appreciated.

Answer 1

Which operating system are you using?
I'm using Mac Lion V 10.7.5, working on a photoshop cs4 trial version (could be the problem?).
I Used the quick mask mode to mask the area.
Here's a screen shot - (I have blacked out some of photo for subjects' privacy):
-  mask mode turned off, then Select>Inverse to show unmasked area  (note, the dotted line continues round the bottom of the pic which indicates to me I have made a masking mistake, as dotted line to mask out background only should go round the top & sides of the photo and then outline the couple, - right?
Image mode of image: If I understand correctly what this is, the image mode is .jpg
Bit depth of image: Bit depth of image is 8, color mode is RGB.
Hope I've given you the information you required.
Thanks
P