Hi Vincent
Step sequencer is currently in alpha testing. It can be accessed by switching your Zynthian to use the feature/stepseq branch from webconf->software->repositories then performing an update. Note that this is cutting edge so may break the Zynthian. It is not yet recommended for production use. I expect this to move to a beta stage within the next few days (or hours @jofemodo?) and appear in the stable release. It is still under development with much work yet to do.
LV2 plugins may be enabled or disabled in webconf->software->LV2 plugins.
Cheers