You need to have transaction batches added and posted to see an amount available for a loan. If you added the new loan prior to adding the transaction batches, you may need to delete the loan and re-add.
If you don't want to add the batches you can override the available amount - click on the 'Review/Edit' link on the New Loan screen and enter the vested balance by source. You can also enter the highest balance in the last 12 months and the current loan balance on this screen. Note that you do need to have sources and a loan fund set up to do this.