Pagerank is a result of the links pointing to your website / directory. Simply work on getting links by either getting good unique content / idea on your directory and blogging about it and announcing it on forums or you can exchange / purchase links (which is actually forbidden, but everyone are still doing it).
Link building (keep in mind that PR is not about the number of links - it's about Quality) and site promotion
a link to your directory is not a bad idea to post. hmm, like aira said, all you need is promotion to get noticed by sites who needs your services. for a directory site, I don't really have the idea on how to gather up quality links on a directory site, but my guess is through advertising it. I hope your directory is not a paid directory.