Multiple Upload is disabled along with the Explorer option on Document Libraries.
This problem is usually caused by a mismatch with Office and Internet Explorer. If you are workign on a 64bit version of Windows you will have two Internet Explorer links, one which opens the 32bit version (x86) and another which will open the 64bit version. The client integration options will only be available in the version which matches your office bit version. For example if you have 32bit office installed you will need to use the 32bit shortcut for internet explorer in order to use the mutliple upload and explorer options.