Please download the latest dev version of CRISPRseek (>= Version: 1.27.5), which has the CRISPRscan gRNA efficacy algorithm implemented. You can download the latest version using git clone firstname.lastname@example.org:packages/CRISPRseek or wait for it to be propagated to http://bioconductor.org/packages/devel/bioc/html/CRISPRseek.html.
Please let me know if it works as expected. Thanks!
Best wishes, Julie