You can only map one column from your source spreadsheet to one field in Salesforce. Once a mapping is created for a particular column, that column is no longer available (i.e. you can't map one column to two fields in Salesforce). In your case, you are already mapping the Mobile column from your spreadsheet to the custom Mobile__c field in Salesforce. It appears that custom field is unnecessary because there already is a standard field for MobilePhone on the Lead object.

To map to the standard field, just map MobilePhone to Mobile from your spreadsheet.

Check is field level security. It's possible that the creator of the Mobile__c field could have hidden the standard MobilePhone field using Field Level Security: https://help.salesforce.com/HTViewHelpDoc?id=users_fields_fls.htm&language=en_US