Typically you don't. You can add a table hint for experimentation using WITH INDEX(1) (similar to adding with NOLOCK) and see if that cures the actual problem. Don't trust the percentage costs in the execution plans, they lie like rugs. Knowing what it's doing can help though.
Are the parameters the same between your test call in SSMS and the one from SSRS? This may be a case of parameter sniffing and you possibly want to include an 'optimize for' selection in the query definition, or use other workarounds to it, such as transferring your parameters to local variables before usage.
Never stop learning, even if it hurts. Ego bruises are practically mandatory as you learn unless you've never risked enough to make a mistake.
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