You are probably not provisioned for MMS service with your provider. You would need to contact your mobile operator to get MMS service enabled for your account.
As for problems sending ... every mobile operator has different settings that are required to be configured in the mobile phone in order to support MMS. There are no magic settings that work with all operators.
So you need to find out what the appropriate settings for MMS are for your operator. A general description of the type of settings information you are looking for can be found here: http://www.nowsms.com/discus/messages/12/4134.html
You must then configure those settings in your mobile phone. And the process of configuring those settings is different for every mobile phone. So you'd need to refer to the manual for that phone to determine how to configure the operator's MMS settings in the phone.
There is a link for documentation/manuals for Motorola phones at the following URL: http://direct.motorola.com/eng/manuals.asp?country=GBR&language=ENG&productid=29 128&cat=3
Unfortunately, the Motorola manual for the v500 doesn't explain how to configure the MMS settings in the phone. For some operators, in some countries, you can use the following service to send the settings to your Motorola phone over SMS: https://secure.mouse2mobile.com/clients/hellomoto/default.asp
Unfortunately, I don't know what else to do to help you, as I do not know exactly what menu option to use on that phone to configure the MMS settings.