Mixed Models: challenge to solve

Hello there!

I have a question concerning my data and the use of Mixed models in SPSS 21.0.
I want to investigate the influence of the independent variable 'altitude' (coded 1 to 5) on the dependent variable 'time' (numeric) of a certain test. This test is conducted in 16 individuals and consists of 100 additions. So, for each addition, the individual needed a certain time period.
This means my datasets compiles following variables:
-testperson(1 to 16)
-altitude (1 to 5)
-addition-number (1 to 100)
So, each test-person conducted 100 additions on each altitude. These 100 additions all needed a specific time to conduct them.
Now, I want to see the results of the times in function of altitude.
For this, I plan to use mixed models with repeated measurements.
- subject: testperson
- repeated measurement: altitude
- covariance structure: unstructured
- dependent variable: time
- factor: altitude
- fixed effect: altitude
- random effect: proband

Now my question: were do I need to put the 'addition number'? This is not a subject, I suppose? Perhaps it is a repeated measurement with a 'random effect'?
-When I put it nowhere: "Warnings - The levels of the repeated effect are not different for each observation within a repeated subject. - Execution of this command stops."
-When I put it in repeated measurements: "Warnings - Insufficient memory to estimate the model parameters. - Execution of this command stops."