If I read your post correctly, you're entering SQL code directly into the Value definition of your variable, which you can't do in SSIS. You can, however, modify your code in the SQL Task to read as such:
DECLARE @dt DATETIME
SET @dt = GETDATE()
Exec dbo.spDoSomething @dt
hth,
Tim
Tim Mitchell, Microsoft Data Platform MVP
Data Warehouse and ETL Consultant
TimMitchell.net | @Tim_Mitchell | Tyleris.com
ETL Best Practices