fpga
Hi,
You'll have to first select the menu, the one which you use for creating new VHDL source code or test bench, there you should see something like Constraint File or something in relation to that...
Then once this process is done,, on the left hand side where you see menu's for design such as Implementation (Translate , Mapping , PAR ) and Generate Programming File over there only I think it's the first optionand under its sub section u can select "Pin Constraint Editor" then you should see a graphical user interface where you can allocate pins manually and allocate various other things as well..
Hope it helps
Sanjay