The taxonomy is still in tech preview - have you switched it on using the below link to the documentation. Do you see a 'Taxonomy' tab in the UI?
https://docs.hortonworks.com/HDPDocuments/HDP2/HDP-2.6.1/bk_data-governance/content/atlas_enabling_t...
Just a word of caution - in a large production environment switching the taxonomy on can cause Atlas performance issues. We switched it off until it comes out of tech preview.
Here's some info on how to query terms via curl. https://atlas.apache.org/0.7.1-incubating/AtlasTechnicalUserGuide.pdf I can't find any info about the terms in the latest rest api documentation.