Whoops, braindead moment on my part. You should still be able to create your own constant with the value TRUE (I would just call the constant 'True' as well, though this isn't necessary) and the type Boolean. Then when you are asked to apply a value, just apply that constant.
In the desktop designer you can create a variable of type Boolean and set it to "true"/"false"
All Answers
Use the Global Constant True as the value you are assigning to that field in the Data Update. More data on this can be found here:
https://na5.salesforce.com/help/doc/en/vpm_designer_resources_globalconstants.htm
I'm using the Desktop flow designer (not the cloud one), so I don't think they have global constants there...
In the desktop designer you can create a variable of type Boolean and set it to "true"/"false"
actually this is what I did and it worked, thanks!
as i mentioned, it did worked, but I encounter another problem with this...
This field should trigger a workflow (If checkbox is TRUE, then date field1 = today and date field 2 = today+90 days)
the workflow is not working..any chance why it is happening?