The "Compute Variable" dialog box is composed of several main areas.
On the upper left, there is a field labeled "Target Variable." Here you would type the name
of the variable that you wish to create. To get to this box, you can hit Alt+T. In this particular
example, we entered the variable salchng in this box.

On the upper right, there is a field labelled "Numeric Expression" where you can type the
expression  that will create the new variable. To get to this box, you can hit Alt+E.
In this example, we entered the expression salary - salbegin. 
To execute this command, hit tab twice, or until the OK box is selected, then hit Enter.
Or, you may hit Alt+P which will paste the syntax into the syntax window and close the dialog box.

If you wish, other portions of the Compute Variable dialog box can be used to create the 
Numeric Expression. For example, rather than typing in the variable salary, you could select
the variable salary from the list of variables in the lower left-hand corner of the dialog box,
then hit Tab twice until the arrow is selected, then Enter. This would move the variable salary into the Numeric Expression box.
In addition, there is a list of mathematical function keywords available in the middle
right-hand side of the dialog box. You can scroll down these functions
(for example, ABS, ANY, ARSIN) until you find one you wish to use; you can select the function,
hit Tab once, and then Enter. This would move the selected function into the dialog box.

If you wish to use the IF function (described later in the SPSS document), you can get to it by
hitting Alt+I.