I’m not sure you are going to get this done with the decarative tools. The box for Allocations is expecting a single field value to be passed to it using skuid’s mustache merge syntax. I think you are going to have to go to Javascript, build your comma delimited list of values and pass this off to your Apex action. Sorry.
(heres a post about building the list - in this case to send a Marketo mass email, but I think you can work out the differences: https://community.skuid.com/t/how-to-add-marketo-send-email-mass-action-to-a-skuid-tab…)
Cheers.
