I am using the prompt for user input data action to create a macro which will generate a powerpoint presentation and supporting documents for talks I give. With this there will be times where I need to enter over 30 variables (I know it's a lot). Having entered the space for these I can only see about 28 of these fields and there doesn't seem to be a way to scroll down to the remaining fields or then to be able to click ok.
No, there is no way to scroll such a prompt. You will need to either reduce the number of entries, or use two successive prompts, or use some other input method (eg the Custom HTML Prompt).
As @peternlewis mentioned, I don't think there is a good solution using only KM UI.
Since you have PowerPoint, then it's likely that you also have Excel.
One approach you might consider is designing an Excel workbook with all the data you need (including default values), and use that as the input to your KM macro to automate PowerPoint.
As I write this, it also occurs to me that it might be better to use MS VBA macro entirely for this workflow. I have found, in general, that for workflow automations that use only MS Office apps that VBA works the best. I do have to say that PowerPoint VBA, while available, is NOT as easy to use as Excel and Word. But I assume you much already have figured this out to automate PowerPoint from KM.
Of course, you also use AppleScript to automate both Excel and PowerPoint. In fact, it may be easier to automate PowerPoint with AppleScript than with VBA.
Anyway, just some ideas that might help.
Good luck, and let us know how it goes.