In order to associate a splash page to an SSID, do the following steps:
STEP 1
Publish your Splash Page (see instructions here:
How to save and publish, or rename the splash page)
STEP 2
Get access to the Splash Page tab of your SSID (if needed, refresh the page to update the drop-down menu from witch you can choose your Splash Pages).
STEP 3
Select from that drop-down menu the Page that you want to use.

STEP 4
Click the "Apply" button at the bottom of the page

Once the configuration is updated, you can already check your results with the
"Simulation Link" in the same tab.