The Dropbox application may be prevented from installing or from syncing over corporate networks. If you are not sure if Dropbox is supported on your corporate network, contact your company's network administrator to find out.
Some corporate networks are behind HTTP proxy servers that use NTLM authentication. Dropbox currently doesn't support this kind of proxy authentication. However, some users have reported some success using an intermediate proxy, such as NTLMAPS or cntlm, to get Dropbox running on their network.