leaving a trailing slash will tell you that the API connection works but won't let you upload contacts. The best way to solve this I found it to change the code in the various functions in HighriseAPI from
$curl = curl_init($this->highrise_url.'
$curl = curl_init(untrailingslashit($this->highrise_url).'
so that the trailing slash is removed no matter what. this is in every function on that page.