So I've tried installing vmware player 3.1.4 and 3.1.3 and both times had the same problem, when I try to load a VM I get the error "Could not open /dev/vmmon". When I ls /dev/ I can see there is no "vmmon" device present. When I try running:

sudo /etc/init.d/vmware start

I get the output:

Starting VMware services:
  VMware USB Arbitrator                                   [  OK  ]
  Virtual machine monitor                                 [FAILED]
  Virtual machine communication interface                 [  OK  ]
  VM communication interface socket family                [  OK  ]
  Blocking file system                                    [  OK  ]
  Virtual ethernet                                        [FAILED]

which shows that the Virtual Machine Monitor fails to load. I tried following the advice on this site and ran

vmware-modconfig --console --install-all

I notice during the compilation there are no errors, but at the end I get the message:

Starting VMware services:
   VMware USB Arbitrator                                   [  OK  ]
   Virtual machine monitor                                 [FAILED]
   Virtual machine communication interface                 [  OK  ]
   VM communication interface socket family                [  OK  ]
   Blocking file system                                    [  OK  ]
   Virtual ethernet                                        [  OK  ]
Unable to start services

Out of curiousity I tried:

sudo /sbin/insmod /lib/modules/2.6.18-238.9.1.el5xen/misc/vmmod.ko

But got the error message:

insmod: error inserting 'vmmon.ko': -1 Invalid module format

I have a feeling this may be the root of the problem, but I don't know what could be causing it or how to fix it.