If you do not have enough messages to send out a campaign, you have a few options.
You can delay sending the campaign and wait for your account to refresh with new messages.
To get more messages added immediately to your account, you can go to the My Account link and select the Purchase Additional Messages option, which come in overage bundles of 4,000 messages. Or, you can either re-purchase your current plan or upgrade to a higher plan by going to the Upgrade tab in your account. If you choose to purchase overage messages, your account refresh date does not change. If you choose the plan option, your refresh date changes to one month from the date of purchase.
A third option is to target your campaign to an amount of subscribers that does not exceed your remaining message amount. You can use either the FanRank™ option, custom groups that you have already created, or create a new temporary group made for the purpose of sending the campaign. You can find these option in the Fans tab.