Use the Actions menu in the IR to download to CSV. Then open the resulting file and save as a *.xls*.
Actions --> Download --> CSV
You can enable/disable the Download option in the Actions menu by toggling the options under the Search Bar section of the Report Attributes.
Edit Report Region --> Report Attributes --> Search Bar
if you update to Apex 4.2 you can use the APEX_IR API to determine the current SQL query behind the IR. You can use the query to build your own export routine to Excel. See http://docs.oracle.com/cd/E37097_01/doc/doc.42/e35127/apex_ir.htm#BABCEIGE for details! APEX_IR.GET_REPORT!
We build an implementation for our own export tool. We use existing Excel templates with a placeholder syntax to generate ready-to-use xls(x) workbooks. You can run a demo from here: http://irdemo.jdd-software.com.