The easiest approach is to use the "Frequencies" function. As I
recall
you pick statistics and you can choose "Mean" under "Central
Tendency".
If you need the average value in your dataset for computation you can
use data --> aggregate. If you want to find the average for your
entire dataset you have to add a variable with a constant value (e.g.
value 1 for all cases) and specify that constant as a break variable.
The dialog lets you choose between several options for what you want
to do with the result: add to the active dataset, or to a new dataset
with a specified name etc. Besides computing averages aggregate lets
you compute frequencies, maximum and minimum values etc over columns
in your dataset.