I saw you in chat stating that advertisements were ineffective at this time, due to them being too expensive. That the ads while bringing in a large amount of visitors only resulted in few signups. Well you see there are several problems here that caused this, lets take a look shall we?
Lets first look at your home page.
A nice neat little homepage I'm sure we can agree. However, I've highlighted some problem areas.
First, the textual content. It gives the plot of the game sure, but it doesn't tell you about it. More specifically, we don't know it's a sign up for a game. What we want is for someone to be attracted to sign up. We want to create desire in the mind of this visitor. We want them to think "Oh wow, this sounds amazing, I HAVE TO PLAY THIS". So far they don't even know it's a game.
Now the visitor doesn't know anything about it yet, they see an about link, they click it, oh wait, it leads nowhere. This is my second problem area, if you aren't giving users information about the game on the front page then the about page is essential. My other problem with this is the about and registration links don't look like clickable buttons. They look like computer monitors, these don't look clickable. I'm sure you have some sort of statistics program installed, look at the statistics, are the users click the top links or instead look to the bottom and click the links there. Perhaps your problem is users want to sign up but it's not clear enough for them. Remember, you have to treat every user of the internet as an idiot until you are proven otherwise. Many will not know that those are clickable links.
So my suggestion to you dear administrator is to sort out your homepage. Advertisements will not be effective in the future if you don't fix this fundamental flaw. Try perhaps redesigning the homepage to include screenshots, how the game works, etc. More than a simple "wham, bam, thank you ma'am" login page.
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The Situation
should make a contest or something for who designs the nicest intro page