Q. In all the preaching I have heard, one thing that is common is that one should not be attached to material world in order to get close to Krishna. The question is, even though Arjuna had material attachment, (BG 1.29) Lord Krishna revealed Himself to Arjuna. Why?
Srila Prabhupada writes in regard to Arjuna's bewilderment before the battle: "The popular saying is that a housewife teaches the daughter-in-law by teaching the daughter. Similarly, the Lord teaches the world by teaching the devotee." [SB 1.9.17] Arjuna is a pure devotee, an eternal associate of the Lord; hence there is no possibility of his coming under illusion due to material attachment. Whenever, therefore, a show is made to teach an exalted devotee such as Arjuna, it is to be understood as an arrangement made by the Lord Himself to place him in such illusion for the sake of teaching us through that instance.
Furthermore, even though Arjuna was placed under illusion by the Lord, his qualification for spiritual enlightenment is evidenced by the qualities of compassion, detachment and sense-control he exhibits in presenting his considerations for refusing to fight, much unlike the disposition of Duryodhana, for example, who was simply interested in his personal aggrandizement and didn't care for any religious principles, welfare of citizens or relatives. (See BG 2.6 purport http://vedabase.net/bg/2/6/en) These qualities, of course, are very conducive but insufficient for spiritual progress; but more importantly, Arjuna surrendered and accepted Krishna as his spiritual master.
Just as you have heard, material attachments are certainly stumbling blocks for spiritual advancement, and even a highly virtuous and soft-hearted person may be misled and impeded by them - this was precisely the reason why Krishna chastises Arjuna, that despite his highly learned and virtuous arguments, he was still on the bodily platform rather than spiritual platform.
But the method of overcoming material attachment is the very process that Arjuna rightly adopted: accepting that he was confused on account of his miserly weakness, placing himself humbly before his spiritual master with great faith, seeking instruction and willing to fully accept and execute the instructions given. (Please refer BG 2.7,8 and 18.73)
After hearing the Bhagavad-Gita, Arjuna did not continue to hold on to his material attachments/ conceptions, but was ready to execute Krishna's will - in this way, he was relieved of his temporary illusion and attachment. Contrast this with the material attachment held on by Dhritarasthra who also simultaneously heard the entire Bhagavad-gita as did Arjuna! As mentioned above, Arjuna's so-called illusion and his response to it present us with lessons on how to properly respond to and overcome our own attachments.