Game level is One Mesh

I designed my entire Level in a third party modeling programme, however when i import it into Unity the entire level is one mesh, how can i break up the model into its seperate parts for texturing?

You need to break it up within your external modelling program. Most modelling applications allow you to create separate objects that can be manipulated independently of others. When you do this, Unity will import them as separate objects as well, allowing you to texture them as you please (although texturing is also generally done within your external editor).

Also, most modeling programs allow you to do the texturing inside them, which Unity will import. If your objects are separate in the modeling program, then it is likely that inside Unity's scene view you have an object that has all the other objects as children of it. You can unparent them (but lose the prefab connection) from there.

Hope that helps!

-CJ