Keychain Access Mojave

Welcome to the Apple Support Communities!I understand you are not seeing any of your passwords in Keychain Access on your iMac. KeyChain Access on your computer and iCloud Keychain are two different things. I'll include some resources below for troubleshooting both of these for you.
Keychain Access User Guide - View the information stored in a keychain on your Mac

1. In the Keychain Access app on your Mac, if you don’t see a list of keychains, choose View > Show Keychains.

2. Select the keychain that you want to view.

3. To see more information about an item, double-click it.

   Tip: To view the information in a selected item quickly, click the Info button in the toolbar.

   If the item is a key:

   
   

   The Attributes pane in the Get Info window reveals basic identifying information for an item, such as its name and type.

   If you want to see the password for an item, click “Show password,” then enter the password for the keychain that contains that item.

The Access Control pane in the Get Info window lets you determine whether you need a password to use an item. For example, if you select “Ask for Keychain password” for an item, you have to type the keychain password as well as the item’s password to gain access to that item.

Get help with iCloud Keychain​
Take care.

Having the same problem on my iMac Pro. Yes its iCloud Keychain and it was already stored on the computer pre updating to Mojave. Keychain is working fine on my MacBook Pro.

It seems that its a problem with the app on iMac, because when you click the 'I' info button on iCloud Keychain it shows the popup box for what must be the first entry on the list, but you just can't see that part of the window (see screenshot). Must be a bug! Pls fix Apple!

I just managed to resolve this issue by toggling the "View -> Show Summary" menu item. Still, this looks like a bug as this menu item shouldn't be toggling the entire main view, only the summary area.

Welcome to the Apple Support Communities!

The link you referred to above is for iCloud Keychain, which is different than the Keychain access you are referring to on your Mac. Take a look at the resource below to copy the keychains you see on your MacBook to your iMac.
Keychain Access User Guide - Copy keychains to another Mac
Take care.

Same problem here on Macbook Air, after updating to macOS Mojave the Keychain Access app is no longer rendering its main view (a gray area is shown instead). This seems to be a bug in the UI rendering as I can still access keychain items by entering the item name into the search box and then pressing the "Info" toolbar button.

I'm not sure if this is somehow relevant, but after updating the system I have turned on NSRequiresAquaSystemAppearance setting and I have turned off CGFontRenderingFontSmoothingDisabled setting.