Shape layer 3D won't let me draw path

We need to know a lot more about your comp and your shot. The normal procedure for doing what I think you are trying to do goes something like this:

1. Open the Motion Tracking workspace (because it is more efficient)
2. Select and edited footage layer because you don't want to spend time working on frames you won't use
3. Run Track Camera with the appropriate settings - Advanced Analysis is almost always a good idea
4. Verify the accuracy by checking the Effects Control Panel then
   1. Select at least 3 points and add set the origin and ground plane on a surface that would be a good general ground plane
   2. Use the same points to add a camera and solid (not a null or text layer) to the comp and name the solid Ground plane or something useful
   3. It is a good idea to set a Layer marker at the point where you establish the origin and ground plane and add your camera and reference solid
5. Verify the accuracy of the Ground Plane track
6. Set other solids (you can't see nulls so it is hard to know if they are accurately sticking to the surface
7. Create as many reference solids as you need and then start adding your other 3D elements to the scene and Shift + Parent to snap new layers to the same position and orientation as the reference solids.

I keep all my reference solids in the comp and set them to guide layers. I often apply grids to the solids to help me verify that they are accurately placed. 

The next step is to take a look at your comp in 4 views so you can see what is going on with the camera and all of the reference solids you have added to the scene. The most likely cause of not being able to work with a path on your 3D layer is that the camera is so far away from the comp center that you exceed the maximum dimensions for a layer in AE. There is a fix. I call it normalizing your comp.

You can do this by going to the layer marker where you established the origin and ground plane, adding a 3D "Master" null to the comp, positioning the master null to 0, 0, 0 because that is where the first object you added to the scene when you created the camera is going to be, then parenting all of the 3D layers and the camera to the Master Null and repositioning the null to the center of the comp. That will make new layers that you add to the comp end up in the center of the comp frame when you make them 3D. Sometimes it is also a very good idea to add a comp-sized solid to the comp, make it 3D and check the size of the solid. If it is really tiny then you can run out of real estate quickly. An easy fix for that is to select the Master Null that has all of the other layers parented to it and scale it up until the test solid you created pretty well fills the frame. 

When all of that is done, release the parenting of all the other 3d layers, delete the frame-sized solid you used to test the camera set up and start adding your new 3D layers or add a new shape layer and make it 3D. You should then end up with a shape that is the right size to let you use the pen tool to start drawing a path.  That's my normal workflow. I even have a script for normalizing a camera-tracked shot. 

I should probably make a tutorial.