I installed DSR and immersive weapons.

the thing is, although in the nexus it said the mesh pack was included, left sheath won't show up for modded weapons.(vanilla works fine)

have i done something wrong with the installation or am i missing something?

During the installation of the latest version of DSR, it should automatically pick up which supported mods you have active, plus you can double-check it in the installation windows (i.e. check/uncheck the mesh packs manually). Perhaps you could uninstall/reinstall if you're unsure if IW was checked. Also, you don't mention it, but you also need to run the patcher after the installation - and also make sure that the mods have the correct load order with LOOT.

Hard to troubleshoot with so little info, but that's the gist of it.