By default deflate module is already compiled on Plesk based Linux servers(new versions). You just need to add the rules in Apache to enable/load the deflate module.

You can test if your website already has mod_deflate enabled using the following tool.

http://www.whatsmyip.org/mod_gzip_test/

Check the main httpd.conf file

# grep 'mod_deflate' /etc/httpd/conf/httpd.conf

If the result was LoadModule deflate_module modules/mod_deflate.so, you can continue. If the result was #LoadModule deflate_module modules/mod_deflate.so, please run the following command:

# perl -pi -e 's/# LoadModule mod_deflate/LoadModule mod_deflate/g' /etc/httpd/conf/httpd.conf

Then add this to the httpd.conf file for the site

#
# Deflate output configuration
#
AddOutputFilterByType DEFLATE text/html text/plain text/xml text/css
BrowserMatch ^Mozilla/4 gzip-only-text/html
BrowserMatch ^Mozilla/4.0[678] no-gzip
BrowserMatch bMSIE !no-gzip !gzip-only-text/html

Restart Apache and test from online tools to see if deflate is enabled.

Other configuration

Include this in the conf.d directory. Make sure that the conf.d directory is included in httpd.conf.

# Insert filter
SetOutputFilter DEFLATE

# Netscape 4.x has some problems...
BrowserMatch ^Mozilla/4 gzip-only-text/html

# Netscape 4.06-4.08 have some more problems
BrowserMatch ^Mozilla/4.0[678] no-gzip

# MSIE masquerades as Netscape, but it is fine
# BrowserMatch bMSIE !no-gzip !gzip-only-text/html

# NOTE: Due to a bug in mod_setenvif up to Apache 2.0.48
# the above regex won't work. You can use the following
# workaround to get the desired effect:
BrowserMatch bMSI[E] !no-gzip !gzip-only-text/html

# Don't compress images
SetEnvIfNoCase Request_URI 
.(?:gif|jpe?g|png)$ no-gzip dont-vary

# Make sure proxies don't deliver the wrong content
Header append Vary User-Agent env=!dont-vary

Restart Apache and test from online tools to see if deflate is enabled.

Easy

On a Plesk server, the mod_deflate module is installed by default, however it may be disabled in the Apache configuration file. To enable the mod_deflate module in Plesk edit the Apache configuration file

# vi /etc/httpd/conf/httpd.conf

Search the line that says,

#LoadModule deflate_module modules/mod_deflate.so

and uncomment it i.e. remove the ‘#’ mark

LoadModule deflate_module modules/mod_deflate.so

Now, create a /etc/httpd/conf.d/deflate.conf file. Apache reads all the .conf files from the /etc/httpd/conf.d directory on a Plesk server.

# vi /etc/httpd/conf.d/deflate.conf

and place the following code in it

SetOutputFilter DEFLATE
SetEnvIfNoCase Request_URI  \
\.(?:gif|jpe?g|png)$ no-gzip dont-vary

Save the file and restart Apache.

# service httpd restart

Source: http://linuxhostingsupport.net/blog/how-to-enable-http-compression-on-a-plesk-server