You will have a problem with this because the two parts of an IF must return the same data type.
Try
if isnull({evaluations.termination_date_of_assignments}) then
"Present"
else
ToText({evaluations.termination_date_of_assignments}, "dd/mm/yyyy")
Notes:
1. If you want the expression to return the date in the field if it is not Null, you just need the field name in the Else part
2. The ToText function for Data data allows you to supply a string argument which 'lays out' the date - this one will give the date as (e.g.) 01/10/2009
3. To get help on a particular function, put it into the Build pane in the Formula Editor, click inside the Function name and press the F1 key on your keyboard. You get context-related Crystal Reports help on that Function.
Good luck!