Hello there!
I'm a bit new to Sketchup, however I have been toying around with it.
My question is that my model "looks solid" , however when I try to use the Solid tools (such as Intersect, Trim, etc), it says it cannot be done as one of the things I'm trying to intersect it with is not a solid. It also does not tell me the volume.
I have used the Solid Inspector tool, and see red/yellow lines, yet deleting the red and yellow lines ruins the model.
I will use the finished model with Ponoko for 3D printing, hence the need for it to be "solid".
Any help? 
The model I have made is attached to this post. But, please ignore the small little shape (that one DOES have a volume, however.)