I was able to produce this problem on my local machine and fix it, so it will be fixed in the next release of Event Calendar Pro. The fix is actually very easy and you can fix your current copy using a text editor. The problem seems to be some kind of IE 7 CSS bug.
If you open the file /Events/Controls/EventEditor.ascx in a text editor, you will see this:
<asp:Panel ID="pnlEdit" runat="server" DefaultButton="btnUpdate" CssClass="panelwrapper eventcalendaredit floatpanel">
remove the "floatpanel" css class so it looks like this:
<asp:Panel ID="pnlEdit" runat="server" DefaultButton="btnUpdate" CssClass="panelwrapper eventcalendaredit">
and it will solve the problem with IE 7. The floatpanel class is not really needed there and seems to not play nice with FCKeditor in IE 7.
I've made the same change on the demo.mojoportal.com site and it seems to have solved it there as well.