one problem in simulate IP core.(SDRAM controller)

Hi,everybody.

I creat a new project. And create a sdram controller with the plug-in manager. At last, the manager create a new subfolder named "testbench" . and there is a vhdl file(**_tb.vhd) in it.

I want to simulate the design. But, when I create the waveform file, I cannot find the nodes in **_tb.vhd with node finder. In the dialog box "select hierarchy level", there are only the main project But no component in the new subfolder "testbench". And the **_tb.vhd is the the new subfolder.

What can i do to find the nodes in **_tb.vhd?

thanks.