First, I would write the IF statement to detect whether the PO# is present in the string.
=IF(FIND("PO#",DESCRIPTION),TRUE,FALSE)
If the string "PO#" is not found, then you want it to return an empty string. So,
=IF(FIND("PO#",DESCRIPTION),TRUE,"")
If the string "PO#" does exist, then you want to find the first space after that string.
=IF(FIND("PO#",DESCRIPTION),FIND(" ",DESCRIPTION,FIND("PO#",DESCRIPTION)),"")
Now, you want to use that space position to take the next five characters (your PO number).
=IF(FIND("PO#",DESCRIPTION),MID(DESCRIPTION,FIND(" ",DESCRIPTION,FIND("PO#",DESCRIPTION))+1,5),"")
Assuming I haven't messed up my syntax in there (), that formula will return the five-digit PO number no matter its location in the DESCRIPTION field. If there is no PO number in the DESCRIPTION field, it will return an empty string. That's a long, ugly formula, I know. If you need me to break it down further for you, just ask.