There is no way to specifically do this in SSRS, but it can be accomplished in the SQL query.
You need to generate a temporary table (a permanent table in the database could also be good, if you are going to be doing this alot) with a list of sequential dates in it, and then join this onto the existing data query with an outer join. This way dates with no data will appear as null entries in the table after the join.
For example, if you had the following tables
DateList
Date
----
2011-01-01
2011-01-02
2011-01-03
2011-01-04
2011-01-05
etc...
SalesData
Date Qty
----------------
2011-01-01 5
2011-01-02 4
2011-01-04 7
then you could use the following query to get the sales data with the
null records for days where there are no sales recorded.
select
dl.Date,
sd.Qty
from SalesData sd
left outer join DateList dl on dl.Date = sd.Date
Date Qty
--------------
2011-01-01 5
2011-01-02 4
2011-01-03 (null)
2011-01-04 7
2011-01-05 (null)
etc...
See this question for information on generating a list of dates to join on.