Hi Hayley,
Not sure from your post if you have done all this already, but from the description that you gave, there may be a simple answer to your problem. The production company can add its own credits to the site, if the production was demonstrably broadcastable. I assume you are saying that this has been done, and the basic page template is available to be viewed. All that has happened is that the production company have not seen fit to include you in the cast list - this may be because of time restrictions, because they did not wish to fill in a full cast list at the time they made the entry etc. *You* should be able to add your own credit to this page, which is what the webmaster appears to be talking about. All you need to do is go to the bottom of the page for the page already set up to advertise the production, and click on 'Update' (this isn't necessarily the most obvious route to do, I grant you). At this point, IMDB will allow you to alert them to any 'omissions or mistakes'. The fact that you were cast, but are not credited can be considered an 'omission', and so you can write to IMDB with the details of your part etc. and you should be able to get your credits added to the list. Even when wheels are in motion, this can take a month or two - and, as we have been discussing, IMDB can drag their feet over the update. If you have done this on several occasions, and are still not seeing an update, I am not sure what IMDB are playing at.
If the production company *haven't* actually put a page up, then that is not IMDB's responsibility, and there is little you can do about it. Similarly, it is not the production company's responsibility to get you listed on IMDB (or, at least, perhaps it should be, to make sure all actors are formally credited, but they are unlikely to worry about it). You should, by rights, be able to add your credits subject to verification - and, if the production company won't *verify* them, then you are dealing with another issue entirely (this is probably the sort of issue you could take to Equity!!).
Once you are on the basic listing, your name and part should appear - you won't see a headshot accompanying the link, nor any further information, as you have to subscribe to IMDB resume before you can add those. Some actors bother with this, many don't (including well-known ones!).