Whilst installing Lead Generator should the following issue be displayed it means that there is no Record Type for Leads in this instance. Lead Generator requires a Record Type for Leads to be able to run efficiently.
To resolve the above issue the System Administrator will need to setup a Record Type for Leads within your Salesforce instance. This can be done at the following configuration:
Setup > Build > Customize > Leads > Record Type
Within the above configuration the System Administrator needs to click on the 'New' button and follow the form ensuring 'Lead Process' is set to default and it is enabled for the profiles that are going to use it.