The "Refugee" scenario.

where is this portal?

I wrote these instructions a long time ago and don't remember the details at all. But if I didn't specify where the portal is located, it should be obvious, so I assume that the portal shown in your screenshot is the one. (The yellow creature runs into it.) If your game is freezing, I can only suggest trying patches. Although, as far as I remember, I completed this scenario on version 1.0, without patches, and nothing froze.