I think the default GNU Radio search path is /usr/share/gnuradio/grc/blocks because all existing blocks are stored within this directory. I added the new path into GRC_BLOCKS_PATH variable.
export GRC_BLOCKS_PATH=/usr/local/share/gnuradio/grc/blocks:$GRC_BLOCKS_PATH
Now I can see the howto block in GRC. For a quick test, I connected a constant source 0.5 to the howto module. The output of howto module is 0.25, which is the square of input. Therefore, the gr-howto block functions as expected. However, when I run the GRC, I saw one error message and am not sure what it indicates.
libGL error: failed to load driver: swrast