Converting to Subdirectory from Subdomains

I have recently installed the WP Multisite. During the installation I somehow remembered that wordpress told me that only subdirectories are allowed since the blog is older than a month. But now when I want to add a site to my network, it seems I can only add it as subdomain. I wouldn’t mind to much, but when I then want to view my site that I added, it says site not found. I am confused. Anybody knows what the problem is?