Based on your Halo Reach Service Record, I can safely assume that you do only have a Xbox Live Silver membership at this time which prohibits access to matchmaking. Then, there exists the following methods to obtain Credits outside from the fixed amount given from Campaign Commendations and Halo CEA Terminal codes.
- Credits for time played
- Constant amount of credits given for every minute played
- Maximum payout after 10 mins
- None Matchmaking Weekly/Daily Challenges
The payout for time played in Halo Reach will vary based on rank and here are some of the recorded credit payouts and as you can see, it's extremely low even at the later ranks.
- Recruit: 50 Cr/10 min played
- Legend: 312 Cr/10 min played
- Mythic: 344 Cr/10 min played
Based on this, my method to rank up in Halo Reach with a Xbox Live Silver Membership would be to focus on completing none Matchmaking Daily/Weekly Challenges and to limit and to quit and then resume the play session after 10 min in order to not get credits wasted from time played.