I have enabled translate glossary option, and so there is my glossary in the project. But I can’t open few terms in the editor, even though I open the file in my mac and I see those terms. It is weird that approximately 20% is not available in the editor for translation. How to make them available?
You’d better switch to this custom application 
If some stings are visible and so are not, this means that the file was imported anyway. Check your duplicates settings, most likely there is something like hide and automatically translate all stings.
And some stings in project are uploaded before that glossary + they are untranslated right now. So they are untranslated master string.
Duplicates get the translations from the master string on export, and if there are no translation, they will be not translated as well.
The solution is be to disable the duplicates option (make all stings visible), add the translation to a to this string manually.
Or allow proofreaders to access hidden stings so they can access hidden content in glossary and translate it.