In this forth Alchemy Training Video I’ll show you how easy it is to create a popup window for your plugin.
Alchemy Training Video 4: Creating a popup window from Content Bloom
The code used in this video is also pasted below in the event you’d like to copy this directly into your own project.
Note: Just change the SchemaDocumentation.aspx url 
execute: function (selection) {
var p = this.properties;
var selectedItem = this._getSelectedItem(selection);
var url = "${ViewsUrl}SchemaDocumentation.aspx#selectedItem=" + selectedItem;
var parameters = "width=650px, height=550px";
var args = { popupType: Tridion.Controls.PopupManager.Type.EXTERNAL };
p.popup = $popupManager.createExternalContentPopup(url, parameters, args);
$evt.addEventHandler(p.popup, "close", this.getDelegate(this.closePopup));
p.popup.open();
}