SP and Lion might not be compatible. If Lion is running in 64bit mode, I recommend that you reboot the MBP into 32bit mode by holding down 3 and 2 on the keyboard during the reboot process. Verify that your MBP is in 32 bit mode under About this Mac information.
Then install the version of SP. If SP won't install correctly, you can try a more recent version of SP v4.20 found on the DIR-827 router site. I have this loaded on my MBP on both OSX 10.6 and 10.7.
After SP is installed reboot and go back into 64bit mode on the MBP.