Hey, we're here to help, so get over it already!
Two more tidbits. On your iMac, click on the W-Fi icon in your Menu Bar, and select the network you set up - it could be that simple. If you don't see your network's name right away, allow you iMac a few seconds (you'll see the "Wi-Fi: Looking for Networks..." at the top) and you may see the network you created. If/when you do, click on that network and you should be good to go
And as a secondary measure, on your iMac, open the Airport Utility app. Iff (that's "if and only if") you've found your network and selected it in the first tip above, you'll see your AE. If you haven't selected your network as described above, in Airport Utility, click the "Other Wi-Fi Devices" button in the upper left corner - and the pull-down menu may show your AE.
Keep in mind that your AE has
three possible networks - that's what mine has - the 2.4GHz network, the 5GHz network (if you've enabled it), and a Guest network (if you've enabled it). You need to have your iMac on the same network for the Airport Utility app to "see" the AE (and, of course, utilize your network) - as your iPad apparently is. I use an iPad Air to tweak my network from time to time as well.
In the Network System Preference, you may have only the previous wireless router (System Preferences > Network > Advanced > Preferred Networks) - your iMac could be looking for the wifi router that you used to have and it's not available. Using the first tip above (to use the Wi-Fi Menu Bar widget) to select your new AE could be your "doh" moment of the day.