Gmod Multiplayer Error

One time, i used steam on another pc. then,on my one,gmod multiplayer won’t work.
can anyone help me?
System specs:
Mac OS X 10.6 using Crossover Games
Processor:
Intel Core 2 Duo 2.4 GHz
Graphics:
ATI Radeon HD 2400 XT:

Chipset Model: ATI Radeon HD 2400
Type: GPU
Bus: PCIe
PCIe Lane Width: x16
VRAM (Total): 128 MB
Vendor: ATI (0x1002)
Device ID: 0x94c8
Revision ID: 0x0000
ROM Revision: 113-B2250H-259
EFI Driver Version: 01.00.259
Displays:
iMac:
Resolution: 1680 x 1050
Pixel Depth: 32-Bit Color (ARGB8888)
Main Display: Yes
Mirror: Off
Online: Yes
Built-In: Yes
Display Connector:
Status: No Display Connected

That should be enough for now. any more specs,let me know. :q:

[editline]10:36AM[/editline]

I told this to steam,but they said to go here.

Log out and back into steam.