I'm working in 3DS Max where I have been modeling a skull for my game. However, I've been unable to properly create vertexes to modify my skull. My Skull Object consists of a Basic Sphere with an Edit Mesh modifier, a Symmetry modifier and a Mesh Smooth modifier (The Object and modifiers are in the order I've named them). So far, the only way I've been able to create more vertexes was by using the "Slice Plane" in the Edit Mesh Modifier, but this isn't very efficient because I don't use all the vertexes I create and having this many vertexes will affect my game's performance. My Question: How do I create single vertexes in objects with the modifiers previously mentioned?

You might want to try the Edit Poly modifier, as that has better ways to create vertices and more day-to-day modelling tools. Both Edit modes (Mesh or Poly) are great, but in your situation the Poly would fit best.

Using Edit Poly there are several ways to create vertices. Every Sub-Object of this editing modifier except "Element" has a function called "Insert Vertex".
Choose the sub-object mode you want to insert a vertex onto and push the Insert Vertex button; click on an equivalent element of your mesh to create a vertex at that position (say, picking Insert Vertex while at Edge sub-object mode will only allow you to create a new vertex at edges).
Remember you have the "Ignore Backfacing" checkbox so you don't end up creating vertices in faces or edges that are behind the one you're actually trying to aim.

There are other ways to indirectly create vertices:
- Selecting two edges and clicking "Connect" will create an edge between the two at their middle. If you want it to be offset from their middle you can edit the Slide setting inside Connect's Settings box (right next to it).
- Selecting a Polygon, Edge or Vertex and clicking Tesselate will create more detail around the selected element.
- Extruding, Bridging, Chamfering and Insetting Polygons and Edges.

- Not related to creating vertices but still... inside the "Edit Geometry" group of Edit Poly you can find the Constraints combo-box. Constraints allow you to limit a vertex's, edge's or face's movement to the selected constraining element. If you're moving a vertex under an "edge constraint", you'll only move that vertex along the edge it belongs to - it will only move "inside" this edge, following its direction. This is very useful.

The Slice Plane is probably the last tool you'd resort to; the above is more than enough for you to model away that skull.

Refer to 3DS Max's User Reference for practical information of all that I said above.

Good luck!

Thank you very much for you help!