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I have run into problems using mana from cards like Black Lotus, Dark Ritual, etc. I can't figure out how to use the mana in my mana pull. Since you can't select them when you are casting a spell. Can anyone help?

Use the ability of the mana source (Black Lotus, Gaea's Cradle, Dark Ritual, etc...) first, then immediately after (don't advance phases or the manapool will empty) cast your spell or ability, it should ask you to "Pay: {ManaCost}". Now, click on the manapool to use the mana that has just been put in by the mana source.

By the way, the manapool should be on the far left of your play area.

Say you have 2 Swamp in play and want to cast Bog Wraith.
Just cast Dark Ritual as usual. This puts 3 black mana into your mana pool.
Then cast Bog wraith. You are asked to pay the 3B cost. Click on the manapool, now the payment screen says: Pay Mana Cost: 1. Click the remaining swamp and you have paid the cost.
Black Lotus works the same way.
Note, to use the Black Lotus to pay for something, you must FIRST tap the lotus, just like with Priest of Titania for instance. These cards cannot be tapped while you are paying for a spell.
Unlike Sol Ring, which can be tapped after you cast a spell.

Final examples:
You want to play Bog Wraith with just 2 swamps and a Sol Ring in play.
You click the Bog Wraith and then click the ring and the swamps to pay for it.

You want to play Bog Wraith with just 2 swamps and a Black Lotus in play.
First, tap the lotus for 3 black mana.
Then cast the Bog Wraith and click the mana pool to use the 3 black mana in it. Then tap a swamp to complete the payment for the Bog Wraith.

I probably used far to many words to explain it.

Took too long to type my message, Dennis beat me to it while I was still typing.

Alight thanks guys. Ill try it out next time I get a hand with something like those. Kinda still wondering where the manapool is located though. Ive been playing MTG Forge for about 2 weeks now and never seen it before. Ill keep a look out on the left for it though.

Thanks again!

It's right here:

OOOOO! WOW! Im stupid....I thought he meant on the left panel

Thanks!