When changing the main ns-2 folder, there is a problem doing a reinstall using the ./install command. The tk directory is not updated with regards to the correct folder name. To make this right, dive into the tk8.x.x/unix subdirectory and edit the directory entries in the “Makefile” file. Do remember to correct all entries, also the ones towards the end of the file.
Then jump back to the main ns-2 directory and run the ./install command again. Good luck!