I'm going to assume you want something to get picked each time, so you want their probabilities to reflect their relative frequency values. In that case, in pseudo-code:

loop through the lines
  parse out the key and the frequency
  save the key and frequency as a two-element array inside an array
  keep a running total of the frequencies
set an accumulator to 0
loop through the array
  replace each frequency value with a value representing the frequency
         plus the accumulator, divided by the total of the frequencies
  add the original frequency to the accumulator
get a random positive number less than 1
loop through the array one more time
  if the random number is less than the relative frequency value 
    we've found our key, print it and exit the loop
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If that makes sense to you, try that and ask if you need help, showing the code you have so far.