If you're trying to sync your iphone/iPad to a different computer than the one that you used to import your CD's, you can't (anymore). Now that Apple is selling iTunes Match ($25/yr) you can only access non-iTunes-purchased songs if they are either:
A) on the exact computer that you used to import them into iTunes
or
B) are already synced to your iphone
Now that iTunes Match exists, if you want to move non-iTunes-purchased music that you imported on one computer...to a different computer, you have to buy iTunes Match. Awesome, huh?
Or transfer the itunes library using an external hard drive from the one computer to the other (and then import into iTunes on the new computer), then sync. Or transfer all the CD's to the new computer directly (yeah right!)