According to "Caveat" section you need kernel "rhel5 064.4 or later".
I'm running kernel 2.6.18-164.2.1.el5.028stab066.10 which should be safe?
My problem is that the ppp feature is unknown to vzctl:
[root@centos ~]# uname -a
Linux centos.localdomain 2.6.18-164.2.1.el5.028stab066.10 #1 SMP Sat Dec 12 12:51:59 MSK 2009 i686 i686 i386 GNU/Linux
[root@centos ~]# modprobe ppp_async
[root@centos ~]# modprobe ppp_async
[root@centos ~]# modprobe ppp_async
[root@centos ~]# lsmod | grep ppp
ppp_async 15232 0
ppp_generic 31884 1 ppp_async
slhc 10240 1 ppp_generic
crc_ccitt 6144 1 ppp_async
[root@centos ~]# vzctl set 102 --features ppp:on --save
Warning: Unknown feature: ppp:on
Bad parameter for --features: ppp:on