The 32-bit edition of Visual Build v8 only auto-locates the 32-bit edition of 7-Zip, and the 64-bit edition only auto-locates the 64-bit edition of 7-Zip. You can specify the path+filename of the executable on the Options tab of the action.
http://www.kinook.com/VisBuildPro/Manual/7zipopttab.htm