Yes, you can, but the mod has to be updated to support 1.5. If you want to know if a mod supports 1.5, you can try to find the mod thread from the forums and see if it says anything about current version. The mod also has to be an SMP (Survival MultiPlayer) mod.
How to install SMP mods:
1. Download the mod. There is a list of mods below.
Thanks for the help