I'm on Windows, but I'm pretty sure it would work the same way on Mac.
1) Locate you KSP folder
2) Open the save folder
3) Open the folder with the name of the save where your ships are (the science one)
4) Open the Ships folder
5) Open either the SPH or VAB folder, depending on which building you built your spacecraft in.
6) Copy the .craft file of the ship you want to copy (file name will be the name you gave your ship)
7) Paste the .craft file in the corresponding folder of your target save (the career one)!
The ships will be available in your new save, but you're gonna have to unlock the parts.
Gilead's post in My Spacecrafts? was marked as the answer