Just bought this Ti-89 and I can't graph a Diff Eq. I followed the directions from the manual on page 69 and get an error: undefined variable

Even though implied multiplication is accepted generally by the calculator when it involves a number and variable, it leads to errors when it involves two sybmols,
xsin(x) is interpreted as a function called XSIN not X*SIN(X).
Suppose you have a function y1, function of t and you write ty1 the calculator will generate an undefined variable error: It reads ty1 as a variable.To prevent the error message you should write t*y1, showing the multiplication explicitly.
Similarly if you write y1'=0.001y1(100-t) the expression y1(100-t) is interpreted as a function call: the function y1 is called with argument 100-t. If it is what you want you should explicitly express the multiplication and wrtite y1'=0.001y1*(100-t).

See image below