You need to give a value to your Likert-scale items. That is to say:
Strongly agree = 5
Agree = 4
Neither agree nor disagree =3
Disagree = 2
Strongly disagree = 1
Lets say respondent 1 answered disagree, respondent 2 answered strongly disagree, and respondent 3 answered neither agree nor disagree.
In this case, you have three "converted" values: 2, 1, 3. Its mean or average equals 2.
I believe it is more meaningful to use a different scale to 1,2,3,4,5 for your code. If you use multiples, 2.5, 5.0, 7.5, 10, you get somewhat of a more meaningful scale in my opinion. Although I am unsure whether there would be any serious statistical implications.
To learn more about this technique in SPSS, I recommend:
All the best,