This is easier to do with the CAML query than with the DVWP parameters.
Select the DVWP in SharePoint Designer. Let's say the date to filter by is called "TheDate". In the ribbon, click the Filter command and create a filter for TheDate - greater than or equal to - [Current Date].
On the View ribbon, click the Task Panes drop-down and enable the Tag Properties Task pane, which will display on the right.
Go into Code View and find the SharePoint:SPDataSource tag. Click inside this tag.
The Properties Task pane now shows the properties for the tag. Look for the property called SelectCommand. Its contents looks something like
<View><Query><Where><Geq><FieldRef Name="TheDate"/><Value Type="DateTime"><Today/></Value></Geq></Where></Query></View>
You need to edit the <Today/>
to include an offset of -2 days: <Today OffsetDays="-2"/>
The complete query is:
<View><Query><Where><Geq><FieldRef Name="TheDate"/><Value Type="DateTime"><Today OffsetDays="-2"/></Value></Geq></Where></Query></View>
You may want to copy the contents to Notepad for easier editing, then paste it back into the properties field.
After that, the DVWP shows the desired values.
You can also edit the value right in the code view, but it's a bit unruly, since the brackets are replaced by the lt and gt html codes. It looks like this (after the change)
selectcommand="<View><Query><Where><Geq><FieldRef Name="TheDate"/><Value Type="DateTime"><Today OffsetDays="-2"/></Value></Geq></Where></Query></View>"