Hopefully most of the coasters import ok. The game may have issues if you import an old coaster and the tracks run into each other. The issue with some rides not importing is that the clearances are different in RCT1 and RCT2 so not all old coasters work.
When you hover your pointer over new button, the tool tip reads "Import Track Designs From Previous RollerCoaster Tycoon(r) Games." When you click the button, a list of all the TD6 and TD4 files in the My Documents\RCT3\Coasters folder will be appear.