You can use the following expressions to convert strings to numbers (from the manual):
float(x)
Convert the integer or text x to a float (fractional number). If x is text, non-numeric characters are allowed after the number, but not before. For example float("3.1xx") returns 3.1, but float("xx3.1") returns 0.
int(x)
Convert the float or text x to an integer (whole number). If x is text, non-numeric characters are allowed after the number, but not before. For example int("33xx") returns 33, but int("xx33") returns 0.
So if you want to assign the contents of a textbox object to a variable as a number, use the following action:
Set variable to float(Textbox.Text)
...or...
Set variable to int(Textbox.Text)
...depending upon how you want to store the number.
You can then assign that variable to an array index.
Alternatively you can skip the variable assignment and go straight to the array:
Set Array.at(X,Y) to float(Textbox.Text)