I have the same problem – can’t enable that option for minified JS/CSS files.
Here is what I get from the compatibility test:
Server Modules & Resources:
Plugin Version: 0.9.2.4
PHP Version: 5.2.16 (PHP5 required for Minify, Rackspace CloudFiles, Microsoft Azure support)
Web Server: Apache
FTP functions: Installed (required for Self-hosted (FTP) CDN support)
Multibyte String support: Installed (required for Rackspace Cloud Files support)
cURL extension: Installed (required for Amazon S3, Amazon CloudFront, Rackspace CloudFiles support)
zlib extension: Installed (required for compression support)
Opcode cache: Not installed
Memcache extension: Not installed
HTML Tidy extension: Installed (required for HTML Tidy minifier suppport)
Mime type detection: Not installed (required for CDN support)
Hash function: Installed (hash) (required for NetDNA purge support)
Safe mode: Off
Open basedir: Off
zlib output compression: Off
set_time_limit: Available
mod_deflate: Not detected (required for Page Cache (enhanced mode) and Browser Cache)
mod_env: Not detected (required for Page Cache (enhanced mode) and Browser Cache)
mod_expires: Not detected (required for Page Cache (enhanced mode) and Browser Cache)
mod_headers: Not detected (required for Page Cache (enhanced mode) and Browser Cache)
mod_mime: Not detected (required for Page Cache (enhanced mode) and Browser Cache)
mod_rewrite: Not detected (required for Page Cache (enhanced mode) and Browser Cache)
mod_setenvif: Not detected (required for Page Cache (enhanced mode) and Browser Cache)
which modules should be enabled/detected for this to work?