Spiffy little routine collegeCAD.
And, since you're in the learning process, would you like some constructive criticism? Keep in mind that most of what I say is just my opinion so don't take offense to anything I say.
First of all let's talk about variable names. There's no reason not to make your variable names descriptive. And that goes for LISP as well as VBA or any other laguage you're programming in. So setting a variable and calling it num1 which is actually a letter (A,B,C, etc...) is a bad habit to get into. In fact, it would make more sense to name it comething like column_count since that's what it actually is. Same goes for your other variables.
I say that for two reasons:
1. When someone else looks at your code it will make more sense if the variable names are indicative of what they stand for.
2. A year from now when you go back and look at your code you'll have to study it in order to remember what you were doing. Setting the variable names properly now will save you headaches in the future.
Next, let's talk about comments. I realize that this was a quick throw-together and that's great. But you should always put comments in your code explaining what is going on. That helps out with both of the points I mentioned above. And you'd be surprised... some of the most powerful LISP routines I have were quick 'throw-together' type routines that I needed right then and there. Some time later I would need that routine again and, not commenting those little routines, well that just makes it difficult to find what you're looking for. Visual LISP has some great 'Search' features and commenting little snippets like this could save you tons of time in the future.
Now to your actual code. COLUMNS are the labels going across the top (A, B, C, D, etc...) and ROWS are the numbered labels on the left so the wording in your prompts needs to change to reflect that.
Secondly, should a person want more than 26 COLUMNS they end up getting ...X Y Z [ \ ] ^ - etc...
It shouldn't bee too difficult to modify your routine so that, should the value of num1 be greater than 26 then the character returned is double lettered as in AA, BB, CC, DD etc... or AA, AB, AC, AD etc...
The second option would be easier, in my opinion, and it would conform to actual spreadsheet standards (open up EXCEL and take a scroll to the left and you'll see what I mean).
Anyway, nice job! And I hope I didn't come across too harsh because I sincerely wasn't trying to be. If you're in the learning stages and you pick up a couple of bad habits they'll bite you in the butt some time in the future. And they'll be much harder to change later on.