You can't "import" from the Aperture database. You just import the original files at their present location to LR.
First in Aperture, ensure that all the photos are "referenced". Use the relocate masters command, which moves them to a regular folder on your hard drive.
Now, do you want to keep any keywords and other metadata? There's a menu command Metadata> Write IPTC Metadata to Master.
You should now be able to import those files.
PS I assume Aperture 3