You can not directly use shapefile in ER Mapper in order to create a region inside of a raster file. However you can import the Shapefile into an ER Mapper's vector format - ERV file, and then can use the ERV file for creating region inside a raster.
To import the Shapefile to an ERV file go to ER Mapper > Utility > Import Vector and GIS format > ESRI Shapefile > Import > select the input file > name output file > set projection if required > OK.
To create a region inside of a raster file using a vector file go to ER Mapper > Process > Polygon to region conversion > Vector dataset polygons to regions. It will ask you to input the vector dataset, which should be your ERV file that you imported from a Shapefile. The output raster dataset should be your raster image data where you want to create the region so that you can clip it. Click OK and it will create a region inside your raster image data.
You must name the region that is residing inside the raster image. Go to ER Mapper > Edit > Edit/Create Region > OK. In the Tools dialog click the Pointer (Select/ Edit points mode) and it will select the region you created from the Shapefile. Then click the "Display Edit Object Attributes" button > name your region and click Save. Now you can use the ClipRegion wizard and clip the image.