The Out of Office Assistant is only available in Corporate/Workgroup Mode.
If you are running Outlook in C/W mode, but the Out of Office Assistant is still missing, here is what to do.
First of all, make sure that the Out of Office Assistant add-on is installed. To do this, select Tools > Options. Click the Other tab, click the Advanced button and then the Add-In Manager. If Exchange Extensions Command is listed, tick the nox in front of it. If it's not listed, click Install. Locate Emsuix.ecf, click OK until you're back at the main Outlook screen. Restart Outlook - you should now be able to see the Out of Office Assistant.
If Out of Office Assistant refuses to install you will need to edit the registry:
Find HKEY_LOCAL_MACHINE/SOFTWARE/MICROSOFT/EXCHANGE/CLIENT/EXTENSIONS. Go to the Edit menu and select Add Value.
Value Name: Exchange Extensions
Data Type: REG_SZ
Value: 4.0;emsuix32.dll;7;011111111111110;1111011100
You're best off doing a copy > paste of the value...
Anyway, hope that helps,
Charlotte