You need to edit the wall by editing the Type Properties. In that dialog box, click on the 'Edit' button next to the field that reads 'Structure' (the top line of the dialog box parameters). On that next window, click the 'Preview' button on the lower left to expand the view of the wall sample within the dialog box. A window will show up to the left and there will be a plan view. Beneath this plan view window will be a button named 'View' Click next to it to switch from the Plan view to the Section view. This will then bring up a section of the wall showing the wall layers.
When you switch to Section, 6 buttons will become selectable on the right window. Click on the 'Modify' button and then zoom to the bottom part of the sample wall section in the left window (you may need to zoom first before clicking the modify button). Once you click 'Modify', then click on the bottom line of the wall layer that you want to adjust - in your case, it will be the layer of the interior wall finishes. Once that is done, a little blue lock will appear. Click the lock to unlock it - it will then look like an unlocked lock. Click accept or OK to get back to your model.
You now have the ability to adjust the bottom of that wall layer in your real building section. In your section that you cut, zoom in to see the foundation and the slab. Click the wall and a solid blue arrow will appear at the bottom of the wall layer you unlocked. You can then grab that arrow and slide the wall layer high enough - then use Align to 'snap' it to the floor slab - or slightly above if you want to.
Keep in mind, you can do this for as many wall layers that exist in your wall - but, you can only unluck wall layes that are adjacent to each other - meaning that if you have more than two layers, you cannot unlock the outer-most and inner-most for that particular wall - only layers that are adjacent to each other.
Hope this helps!
