[Plugin: Hide or Show Comments] Cant find single.php

the template file single.php of your theme is under:

/wp-content/themes/{your-theme-name}/single.php

btw:
you can edit theme files from:
‘dashboard’ ‘apperance’ ‘editor’ – then select ‘single.php’

you should not generally need a plugin to control comments;
there are options in:
‘dashboard’ ‘settings’ ‘discussion’
and:
‘discussion’ box below the post/page editor window on the ‘edit post’ or ‘edit page’ pages.