The simplest "pop up window" you might use is Show Custom Dialog. It can have up to three input fields and up to three buttons. It's probably too limited for what you want here as you can't format the data input fields and the buttons can only dismiss the dialog. (Your script can detect which button was pushed and thus do different things depending on what button was pushed, but only after the dialog box has disappeared.)
An alternative is to use New window to open up a small floating window. With this option, you select a fileMaker layout for the window and thus have complete control over what fields, buttons, scripting do when the user interacts with the window.
The implementation details for how best to use New Window depend on what version of fileMaker you have (v12 or an older version?) and what platform (mac or windows?).
how do i tell it not to over write contents already there.
Option 1: Use set field, but include the current value of the field in the "calculated result" parameter:
Set field [YourTable::YourFieold ; YourTable::YourField & "Put new data to be added here"]
Option 2: insert text and Insert Calculated Result can append data to that already in the field. The "insert" script steps fail to work if the target field is not present on the current layout when the step is executed. I thus prefer to use Set Field whenever possible.
How do i get it to put in date and time like this 21/05/12 - 15:30
If you use a time stamp field, you can get something very close, if you specify a - separator as the last item for the date formatting and then select 24 hour format for the time format.
Or you can construct a text calculation that combines date and time while using the specified format.
Or you can put the date in one Merge field, the Time in a second merge field to get something like this:
<<DateField>> - <<TimeField>>
You can select this text block and use data formatting to specify the DD/MM/YY and 24 hour formats just like you can with a time stamp field.