Use Mesh > Patch to fill holes in a mesh with triangular faces.

To patch holes

1. Select the Borders tool from the Select/Move Tools tray.
2. Drag across any border around the hole you want to patch.
3. Select Mesh > Patch from the main menu bar.

   Mudbox fills the hole in the mesh, and the keeps the newly created faces selected so you can continue to adjust the region, for example using Tighten Selection or Fair Selection. See Smooth selected regions of a mesh.