At the bottom of your screen you'll see a series of buttons (Snap-Grid-Ortho...) The last one will say either "Model" or "Paper". If it says "model" then you are in model space even though you're in a layout. Click the button to toggle between Model Space (MS) & Paper Space (PS)
Though some disagree, I agree completely with smcadman - model in MS and dimension in PS.
WRT freeze and layer control, on the "Object Properties" toolbar, at the left, is an icon that appears (to me) to be three pieces of white paper stacked on each other. Click that to open up your layer manager (or type "layer"

.
If you're working in the Model Tab, you'll see that the third column is labeled "Freeze in all VP". That means that you freeze/thaw (for now, just think of it as turning on & off) that layer in any viewport.
If you switch to one of the Layouts, and bring up Layer Manager, you'll find a column which says "Current VP Freeze". That means that the layer will be frozen (off) in the current viewport but visible in all other viewports. That enables you to show entities or dims or whatever only in one viewport but not in the others. (They will ALL be visible in the Model Tab unless you freeze them there in which case they will also be frozen in all the other viewports).