We have a thread about this somewhere, I'm sure. Just about any sim can ask their boss for promotion. You can only ask on certain days, and ambitious and/or workaholic traits increase the likelihood that your boss will say yes. You need to have at least 75% performance bar filled (2/3 if you have the right trait) and be at least good friends with your boss. I always shoot for best friends first by enthusing about their job (eg, enthuse about cooking) because 'enthuse' interactions, if accepted, never get boring. First, you 'discuss work' and then the option should come up. If your sim is ambitious or workaholic, and it doesn't come up under friendly interactions, it will be under the 'special' interactions. You can only ask for a promotion every 3 days.