Hi Dianna D,
After speaking with an employee from Marketo on the phone yesterday, he was able to assist me with this.
What had happened to me was that I currently had an active trigger campaign that was set up to automatically sync any uploaded leads straight to Salesforce. All I needed to do is insert a filter that excluded the list I was about to upload.
So in order, I needed to create a list name in Lead Database, go back to the trigger campaign and include the filter 'Member of List - not in - [and list name]... Go back to Marketing Activities and create a program which you can affiliate the list with and then upload the leads in the List you added. This finally worked for me and will hopefully works for you too.
Goodluck!