How do i convert this C if, elseif, else to Common Lisp?

Here is the statement::

            if( frameCount < 30 ){
        accumulateBackground( frame );
    }else if( frameCount == 30 ){
        createModelsfromStats();
    }else{
        backgroundDiff( frame, mask1 );

i tried this:

      (if (< frame-count 30) (accumulate-background frame) 
      (if (equal frame-count 30) (create-models-from-stats))
      (background-diff frame mask-1))

but i get an invalid number of elements error:

i thought of doing progn but that doesnt represent the right condoitionals...and thought of cond but that will not eval the same way as a if, eleif, else statement...any help is appreciated..

You had the parens wrong in your nested if. Here's a corrected version. Note how the indentation makes it easier to follow.

(if (< frame-count 30)
  (accumulate-background frame)
  (if (equal frame-count 30)
    (create-models-from-stats)
    (background-diff frame mask-1)))

Here's an equivalent COND version.

(cond
  ((< frame-count 30)
   (accumulate-background frame))
  ((equal frame-count 30)
   (create-models-from-stats))
  (t
   (background-diff frame mask-1)))

Disclaimer: neither of these are tested, but they look right.