Only eliminated players can re-buy. (Active players can add-on, and inactive players can buy-in.)
1. To record a re-buy for an eliminated player, tap the player:
2. Tap re-buy in the popup menu:
3. Edit the Chips and Cost values if necessary, then tap Do It!
4. The player is active again, but now they should be placed at a table. You can tap the Edit button (in the upper right), and drag-drop the player to the table and position desired, or you can tap the player:
5. Then tap "Place at table":
6. The app randomly chooses between the tables that have the least number of players at them. If you're happy with the placement, tap Do It!
(If you're not happy with the placement, you can always go back, go into edit mode and drag-drop the player to any table and position you want.)