Christ taught that whatever we want that is good, showing faith in him, we may receive it (see also 3 Nephi 18:20).
Have miracles ceased just because Christ has left the earth? Of course not; his power remains with his disciples. See commentary at Mormon 9:7–20 and “A God of Miracles Still Today,” in commentary at Mormon 9:24.
The phrase “Christ hath ascended into heaven … to claim of the Father his rights of mercy” refers to the Savior’s right and ability to decide who receives the full benefits of the Atonement, including forgiveness of sin, and who does not. This is illustrated by the Savior’s description to Joseph Smith of His advocacy on our behalf:
“Listen to him who is the advocate with the Father, who is pleading your cause before him—
“Saying: Father, behold the sufferings and death of him who did no sin, in whom thou wast well pleased; behold the blood of thy Son which was shed, the blood of him whom thou gavest that thyself might be glorified;
“Wherefore, Father, spare these my brethren that believe on my name, that they may come unto me and have everlasting life” (D&C 45:3–5).
In other words, the fulness of Christ’s mercy will be applied only to those who exercise faith in him, as Mormon taught later on in his sermon. They who have faith in Christ will cleave unto every good thing. See commentary at Moroni 7:15–17.
Christ “advocateth the cause of the children of men.” Christ will be our Advocate and Defender before the Father; see also commentary at 2 Nephi 19:6–7, under “Counselor.”