1) Start in the "Current Stock" menu.
You can see the deactivated SKUs for "Recommended Send-Ins" by the red badge next to your recommended FBA Send-In quantity.
2) Check all the SKUs you want to activate. Click on Actions and then on "Batch edit the selected SKUs".
3) Set "Include SKU in automatic FBA Send-In suggesstions?" to enabled and save your settings.