How do I get my iPhone trust this computer to appear?

You may learn --> About the 'Trust This Computer' alert on your iPhone, iPad, or ...

Change your settings for trusted computers

Your device remembers the computers you've chosen to trust.

If you don't want to trust a computer or other device anymore, change the privacy settings on your iPhone, iPad, or iPod touch. Go to Settings > General > Reset > Reset Location & Privacy. Now when you connect to formerly trusted computers, the Trust alert will ask you whether you trust that computer. 

unlock your i phone while u have connected it to your pc or laptop. U will get a msg on your phone to trust this device or not.