1) Use scp (winscp, pscp) to copy the file(s) to a directory on the server. Some place like root's home directory is sufficient. Generally, there will be more than one file, they'll look something like:

2.1G	centos-5-1-prod-f001.vmdk
2.1G	centos-5-1-prod-f002.vmdk
2.1G	centos-5-1-prod-f003.vmdk
2.1G	centos-5-1-prod-f004.vmdk
2.6M	centos-5-1-prod-f005.vmdk
4.0K	centos-5-1-prod.vmdk

You need to copy all of these files to the server.

2) Login to ESX server (most likely as the root user)
3) To see the volumes on the VMware server, type 'vdf' to see the vmfs volumes, then:

cd /vmfs/volumes/somevol/movedserver

4) Run the following:

vmkfstools -i /path/to/centos-5-1-prod.vmdk centos-5-1-prod.vmdk

5) Once conversion is complete you can delete the original centos-5-1-prod.vmdk and it's associated files.
6) Create a new VM, tell it to use existing and browse to /vmfs/volumes/somevol/movedserver/centos-5-1-prod.vmdk

Taken and edited from here

If you're using IDE drives, you have to use the following: Converting a GSX Server Virtual IDE Disk to a Virtual SCSI Disk.