Select the dimensions, click on the "X", and select the tolerance style you want and then you can enter the tolerance.
as mentioned by @rehenryjr You have to define what type of dimension and/or tolerance You want to have.
Understanding Your last answer it sounds to me as if You first define the type of dim and the tolerance before creating the dimension itself.
I prefer the other way.
First I create my dimensions and when I have them I consider the toleranceing aspect of the part while adding tolerances to the existing dimensions.
Toleranceing not only is a matter of dimensions but also to have GDT frames , datums, surface symbols, and much more