Code Sign error ... doesn't match any valid certificate/private key pair

[TechnoEagle]

I joined the iPhone Developer Program a few weeks ago and have successfully been able to put my apps on my iPod touch from my computer. Now I am also working on my app on my dad's computer, but when I click 'Build and Run' after changing 'Simulator - 4.1' to 'Device - 4.1', I get the following error:

Code Sign error: The identity 'iPhone Developer' doesn't match any valid certificate/private key pair in the default keychain

I know that I have to copy my certificate and provisioning profile to my dad's computer, but I followed an online tutorial about that, so I don't know what the problem is.

If anyone has any suggestions, I would much appreciate it!
Thanks in advance!

[tychop]

In XCodes Organiser, export your Developer certificates & import them via the same user interface on the other machine.