Alternate casting methods?

[EVOLVF]

I can't for the life of me work out how to cast cards that have alternate casting methods. Not sure if it's a glitch or if it's just me being an idiot.

For example, I casted Omniscience but every card I tried to play still required me to play the mana cost.

I wasn't able to play Dread Return with Flashback as it kept asking me to pay the mana cost as opposed to sacrificing creatures.

Same goes for Dredge (the mechanic)!

As I said before, not sure if I just haven't worked out how to use them properly, but it would be great if someone could explain what's going on.

I'm using the latest beta build.

[elcnesh]

It's actually a known issue in the latest beta, and has been fixed in the latest snapshot.