Can any1 tell me if I can enter a spline into AutoCAD using a Formula?

Not directly (using an internal command). But you can construct entities using simple LISP or VBA macros or using scripts generated e.g. from Excel.

I have an excel spread sheet with the different co-ordintaes, so I can use these?
 
Where would I find te LISP command tool?

You don't need a LISP tool for this - just follow the instructions in the tip:
http://www.cadforum.cz/cadforum_en/qaID.asp?tip=6606

Thanks for the tip. That was very helpful.