Just wanted to be sure that we are talking about the same smart list (see screen shot of All Leads system smart list below). If so, when you click the "Leads" tab in a smart list, the message "Loading xx,xxxx" will be displayed briefly before the list is displayed. If this is being displayed, is the number it displays close to the number of records that should be in Marketo?
Are any leads displayed in the list? If so, is the counter at the bottom right displaying a value close to the number of records that should be in Marketo? If not, click the number and it will be recalculated.
Does the View you selected (in the upper right corner of list) have fields that have data in them? if not, it may appear that no leads are being displayed and you should change your view to one with fields that have data in them.
If you still aren't seeing your leads or the count is way off, you should open a case with tech support.