Students can use the onscreen/virtual keyboard (sometimes referred to as 'formula editor') to enter answers. Specific functions, like, log and ln, can be found on the tab 'function'.
Another way of entering answers is using your own keyboard. This way, you can also type functions, for example you can simply type 'ln' or 'log(a,b)' for log_a(b).
The formula editor (Virtual keyboard) may freeze up with long expressions. Try breaking the expressions into smaller parts - it should help speed things up. Also, consider using keyboard input as an alternative.
This is all explained in the page "Getting used to the test software" in the "OMPT // Welcome" chapter.