Looking better now, still few things to mention:

• with my previous post saying if divide by \'2\' doesnt match then all even numbers dont match, thats something you can keep in consideration. Check if it is dividable by 2 and if it isnt, only check UNeven numbers afterwards \'inc %prime 2\'. This will make the loop 2x faster.
• You dont need the if (!%pr) bit because the script should be already halted if a match was found, so you dont need to set the %pr=1 too

• By comparing it like this \'if $calc(xxx) == $int($calc(xxx)) you are actually doing 2 calculations, which slows the script. This script obviously depends on speed, so set a variable, BUT dont use VAR in the loop itself, do it like this:
  var %prime, %calc
  ;restofcode
  %calc = $calc($1 / %prime)
  if (%calc = $int(%calc))
  etc.

• Now you can avoid this, but too be extremely picky, in a single if-statement you dont have to use the { } brackets, and it speeds up processing leaving them away, so it would be:
  else inc %prime
  instead of
  else { inc %prime }