In general, slapd is run like this:

/usr/local/etc/libexec/slapd [<option>]*

Where /usr/local/etc/libexec is determined by configure and <option> is one of the options described above (or in slapd(8)). Unless you have specified a debugging level (including level 0), slapd will automatically fork and detach itself from its controlling terminal and run in the background.