Hey there, Roy!
Thanks for visiting the Apple Support Communities. I understand from your post that when connecting your iPad to your iMac, it isn't working. You select to trust your computer as requested but it still doesn't seem to recognize the iPad. I'm happy to help!
First, check out the information in the Apple Support article About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch - Apple Support, then work through the "Get help" section.
If you need additional assistance, the steps in If iTunes doesn't recognize your iPhone, iPad, or iPod - Apple Support may also provide help.
Should the issue continue after that, please get in touch with Apple Support directly, which you can do here: Contact Apple Support.
Cheers.